don't panic on unsupported type

In Terraform, `make` fails because reflectwalk occurs panic.

    --- FAIL: TestApply_vars (0.00 seconds)
    panic: unsupported type: invalid [recovered]
            panic: unsupported type: invalid
    
    goroutine 168 [running]:
    runtime.panic(0x5c18a0, 0xc2080fa240)
            /usr/lib/go/src/pkg/runtime/panic.c:279 +0xf5
    testing.func·006()
            /usr/lib/go/src/pkg/testing/testing.go:416 +0x176
    runtime.panic(0x5c18a0, 0xc2080fa240)
            /usr/lib/go/src/pkg/runtime/panic.c:248 +0x18d
    github.com/mitchellh/reflectwalk.walk(0x0, 0x0, 0x0, 0x0, 0x62da80, 0xc208090310, 0x0, 0x0)
            /tmp/gopath/src/github.com/mitchellh/reflectwalk/reflectwalk.go:124 +0x2ba
    github.com/mitchellh/reflectwalk.Walk(0x0, 0x0, 0x62da80, 0xc208090310, 0x0, 0x0)
            /tmp/gopath/src/github.com/mitchellh/reflectwalk/reflectwalk.go:74 +0x19f
    github.com/hashicorp/terraform/config.(*Config).Validate(0xc20804a400, 0x0, 0x0)
            /tmp/gopath/src/github.com/hashicorp/terraform/config/config.go:130 +0x22fb
    github.com/hashicorp/terraform/command.(*Meta).Context(0xc2080905b0, 0xc20801bc20, 0x4f, 0xc2080fa430, 0x10, 0x17, 0x64d8b0, 0x0, 0x0)
            /tmp/gopath/src/github.com/hashicorp/terraform/command/meta.go:91 +0x4c3
    github.com/hashicorp/terraform/command.(*ApplyCommand).Run(0xc2080905b0, 0xc20801bcb0, 0x1, 0x1, 0x989570)
            /tmp/gopath/src/github.com/hashicorp/terraform/command/apply.go:71 +0x5ff
    github.com/hashicorp/terraform/command.TestApply_vars(0xc208049200)
            /tmp/gopath/src/github.com/hashicorp/terraform/command/apply_test.go:604 +0x27f
    testing.tRunner(0xc208049200, 0x97adc8)
            /usr/lib/go/src/pkg/testing/testing.go:422 +0x8b
    created by testing.RunTests
            /usr/lib/go/src/pkg/testing/testing.go:504 +0x8db