cmd/compile: nil deref in error message

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
