cmd/compile: nil deref in error message

[dvyukov]

gc produces the following error message on the program:

package a
func n() {(interface{int})}

go.go:2: type %!v(PANIC=runtime error: invalid memory address or nil pointer dereference) is not an expression

The error message needs to be fixed.

go version devel +ef37184 Mon Jul 6 00:07:10 2015 +0000 linux/amd64

[gopherbot]

CL https://golang.org/cl/14015 mentions this issue.