runtime: be very careful with bad pointer tests

Saw this on a test:

runtime: bad pointer in frame runtime_test.testSetPanicOnFault at 0xc20801c6b0: 0xfff
fatal error: bad pointer!

runtime stack:
...
copystack(0xc2081bf7a0, 0x1000)
        /root/work/solaris-amd64-smartos-2dde8b453d26/go/src/runtime/stack.c:621 +0x173 fp=0xfffffd7ffd5ffee0 sp=0xfffffd7ffd5ffe20
runtime.newstack()
        /root/work/solaris-amd64-smartos-2dde8b453d26/go/src/runtime/stack.c:774 +0x552 fp=0xfffffd7ffd5fff90 sp=0xfffffd7ffd5ffee0
runtime.morestack()
        /root/work/solaris-amd64-smartos-2dde8b453d26/go/src/runtime/asm_amd64.s:324 +0x90 fp=0xfffffd7ffd5fff98 sp=0xfffffd7ffd5fff90

goroutine 163354 [stack growth]:
...
runtime.convT2E(0x587000, 0xc20807bea8, 0x0, 0x0)
        /root/work/solaris-amd64-smartos-2dde8b453d26/go/src/runtime/iface.go:141 +0xd2 fp=0xc20801c678 sp=0xc20801c640
runtime_test.testSetPanicOnFault(0xc20822c510, 0xfff, 0xc20801c748)
        /root/work/solaris-amd64-smartos-2dde8b453d26/go/src/runtime/runtime_test.go:211 +0xc6 fp=0xc20801c718 sp=0xc20801c678
...

This test is testing bad pointers.  It loads the bad pointer into a pointer variable,
but before it gets a chance to dereference it, calls convT2E.  That call causes a stack copy,
which exposes that live but bad pointer variable.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/146880043