Repository: go
Issue: unique: `fatal error: getWeakHandle on invalid pointer` with `sbrk=1`
Labels: NeedsInvestigation,compiler/runtime

### Go version

go1.24-0206eb9679, 1.23.1

### Output of `go env` in your module/workspace:

```shell
GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/cbrasic/Library/Caches/go-build'
GOENV='/Users/cbrasic/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/cbrasic/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/cbrasic/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/cbrasic/repos/goroot'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/cbrasic/repos/goroot/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='devel go1.24-0206eb9679 Mon Sep 30 19:15:39 2024 +0000'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/cbrasic/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_n/xv5mgbd173xf6p4vwlq3hmpw0000gp/T/go-build1074858196=/tmp/go-build -gno-record-gcc-switches -fno-common'
```


### What did you do?

I noticed that my go1.23.1 app panics when run with `GODEBUG="sbrk=1"` and tracked it down to `net/netip`, specifically [this line](https://github.com/golang/go/blob/bae2e968e2daadd39c1bdb1221648361d7277ddc/src/net/netip/netip.go#L71). I saw https://github.com/golang/go/issues/69458 and thought this issue might already be fixed so I rebuilt with go master but saw the same issue. Any app that imports `net/netip` will crash when run with `GODEBUG="sbrk=1"`

### What did you see happen?

```
$ cat main.go
package main

import "unique"

type addrDetail struct {
        isV6 bool
}

var (
        z6noz = unique.Make(addrDetail{isV6: true})
)

func main() {
}
$ go build main.go; GODEBUG="sbrk=1" ./main

fatal error: getWeakHandle on invalid pointer

goroutine 1 gp=0x10b290030 m=0 mp=0x104720320 [running, locked to thread]:
runtime.throw({0x10468f442?, 0x10462a4dc?})
        /Users/cbrasic/repos/goroot/src/runtime/panic.go:1069 +0x38 fp=0x1400008cc20 sp=0x1400008cbf0 pc=0x10467f038
runtime.getWeakHandle(0x10b2d5e58)
        /Users/cbrasic/repos/goroot/src/runtime/mheap.go:2157 +0x138 fp=0x1400008cc70 sp=0x1400008cc20 pc=0x104642d58
runtime.getOrAddWeakHandle(0x10b2d5e58)
        /Users/cbrasic/repos/goroot/src/runtime/mheap.go:2098 +0x20 fp=0x1400008ccc0 sp=0x1400008cc70 pc=0x104642a50
internal/weak.runtime_registerWeakPointer(0x1400008cd01?)
        /Users/cbrasic/repos/goroot/src/runtime/mheap.go:2045 +0x1c fp=0x1400008cce0 sp=0x1400008ccc0 pc=0x10467e95c
internal/weak.Make[...](0x10b2d5e58?)
        /Users/cbrasic/repos/goroot/src/internal/weak/pointer.go:62 +0x74 fp=0x1400008cd00 sp=0x1400008cce0 pc=0x10468acb4
unique.Make[...].func1()
        /Users/cbrasic/repos/goroot/src/unique/handle.go:64 +0xb4 fp=0x1400008cd50 sp=0x1400008cd00 pc=0x10468abe4
unique.Make[...]({0x0})
        /Users/cbrasic/repos/goroot/src/unique/handle.go:74 +0x144 fp=0x1400008cdf0 sp=0x1400008cd50 pc=0x10468aab4
main.init()
        /Users/cbrasic/repos/test/main.go:10 +0x28 fp=0x1400008ce10 sp=0x1400008cdf0 pc=0x104689828
runtime.doInit1(0x10471c350)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:7303 +0xd4 fp=0x1400008cf40 sp=0x1400008ce10 pc=0x10465fd44
runtime.doInit(...)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:7270
runtime.main()
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:254 +0x340 fp=0x1400008cfd0 sp=0x1400008cf40 pc=0x104650790
runtime.goexit({})
        /Users/cbrasic/repos/goroot/src/runtime/asm_arm64.s:1223 +0x4 fp=0x1400008cfd0 sp=0x1400008cfd0 pc=0x1046847e4

goroutine 17 gp=0x10b2d52f8 m=nil [force gc (idle)]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:435 +0xc8 fp=0x1400002e790 sp=0x1400002e770 pc=0x10467f118
runtime.goparkunlock(...)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:441
runtime.forcegchelper()
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:348 +0xb8 fp=0x1400002e7d0 sp=0x1400002e790 pc=0x104650a28
runtime.goexit({})
        /Users/cbrasic/repos/goroot/src/runtime/asm_arm64.s:1223 +0x4 fp=0x1400002e7d0 sp=0x1400002e7d0 pc=0x1046847e4
created by runtime.init.7 in goroutine 1
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:336 +0x24

goroutine 18 gp=0x10b2d5548 m=nil [GC sweep wait]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:435 +0xc8 fp=0x1400002ef60 sp=0x1400002ef40 pc=0x10467f118
runtime.goparkunlock(...)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:441
runtime.bgsweep(0x10b2d54c0)
        /Users/cbrasic/repos/goroot/src/runtime/mgcsweep.go:277 +0xa0 fp=0x1400002efb0 sp=0x1400002ef60 pc=0x10463cf10
runtime.gcenable.gowrap1()
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:203 +0x28 fp=0x1400002efd0 sp=0x1400002efb0 pc=0x1046310f8
runtime.goexit({})
        /Users/cbrasic/repos/goroot/src/runtime/asm_arm64.s:1223 +0x4 fp=0x1400002efd0 sp=0x1400002efd0 pc=0x1046847e4
created by runtime.gcenable in goroutine 1
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:203 +0x6c

goroutine 19 gp=0x10b2d5728 m=nil [GC scavenge wait]:
runtime.gopark(0x10b2d54c0?, 0x1046a8e88?, 0x1?, 0x0?, 0x10b2d5728?)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:435 +0xc8 fp=0x1400002f760 sp=0x1400002f740 pc=0x10467f118
runtime.goparkunlock(...)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:441
runtime.(*scavengerState).park(0x10471fba0)
        /Users/cbrasic/repos/goroot/src/runtime/mgcscavenge.go:425 +0x5c fp=0x1400002f790 sp=0x1400002f760 pc=0x10463a9ac
runtime.bgscavenge(0x10b2d54c0)
        /Users/cbrasic/repos/goroot/src/runtime/mgcscavenge.go:653 +0x44 fp=0x1400002f7b0 sp=0x1400002f790 pc=0x10463aee4
runtime.gcenable.gowrap2()
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:204 +0x28 fp=0x1400002f7d0 sp=0x1400002f7b0 pc=0x104631098
runtime.goexit({})
        /Users/cbrasic/repos/goroot/src/runtime/asm_arm64.s:1223 +0x4 fp=0x1400002f7d0 sp=0x1400002f7d0 pc=0x1046847e4
created by runtime.gcenable in goroutine 1
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:204 +0xac

goroutine 20 gp=0x10b2d5b40 m=nil [chan receive]:
runtime.gopark(0x14000032738?, 0x1046505c4?, 0x38?, 0x27?, 0x0?)
        /Users/cbrasic/repos/goroot/src/runtime/proc.go:435 +0xc8 fp=0x140000326f0 sp=0x140000326d0 pc=0x10467f118
runtime.chanrecv(0x10b2d5ac0, 0x0, 0x1)
        /Users/cbrasic/repos/goroot/src/runtime/chan.go:640 +0x408 fp=0x14000032770 sp=0x140000326f0 pc=0x104624c88
runtime.chanrecv1(0x104720320?, 0x104650850?)
        /Users/cbrasic/repos/goroot/src/runtime/chan.go:490 +0x14 fp=0x140000327a0 sp=0x14000032770 pc=0x104624874
runtime.unique_runtime_registerUniqueMapCleanup.func1(...)
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:1731
runtime.unique_runtime_registerUniqueMapCleanup.gowrap1()
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:1734 +0x3c fp=0x140000327d0 sp=0x140000327a0 pc=0x104633f8c
runtime.goexit({})
        /Users/cbrasic/repos/goroot/src/runtime/asm_arm64.s:1223 +0x4 fp=0x140000327d0 sp=0x140000327d0 pc=0x1046847e4
created by unique.runtime_registerUniqueMapCleanup in goroutine 1
        /Users/cbrasic/repos/goroot/src/runtime/mgc.go:1729 +0xa0
```

### What did you expect to see?

no crash