cmd/gc: more than one recursive type references causes segmentation fault

go version go1.4 darwin/amd64

This:

```
type foo struct {
    bar foo
}
```

produces the expected error message:

```
invalid recursive type foo
```

But if more than one recursive reference is present:

```
type foo struct {
    bar foo
    blah foo
}
```

then it crashes, rather than reporting the error as expected:

   /usr/local/go/pkg/tool/darwin_amd64/6g: signal: segmentation fault
