cmd/gc: don't dereference a nil Type pointer in widstruct

The nil dereference in the next few lines doesn't seem
to cause a segmentation fault on Unix, but does seem
to halt the Go compiler.

The following is a test case:

>>>
package main

func mine(int b) int {
        return b + 2
}

func main() {
        mine()

        c = mine()
}
<<<

Without this change only the following is caught:

typecheck.go:3: undefined: b
typecheck.go:4: undefined: b

with it, we catch all the errors:

typecheck.go:3: undefined: b
typecheck.go:4: undefined: b
typecheck.go:10: undefined: c
typecheck.go:10: cannot assign to c .

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6542060

src/cmd/gc/align.c

@@ -54,6 +54,8 @@ widstruct(Type *errtype, Type *t, vlong o, int flag)
 	for(f=t->type; f!=T; f=f->down) {
 		if(f->etype != TFIELD)
 			fatal("widstruct: not TFIELD: %lT", f);
+		if(f->type == T)
+			break;
 		dowidth(f->type);
 		if(f->type->align > maxalign)
 			maxalign = f->type->align;

test/typecheck.go

@@ -0,0 +1,18 @@
+// errorcheck
+
+// Verify that the Go compiler will not
+// die after running into an undefined
+// type in the argument list for a
+// function.
+// Does not compile.
+
+package main
+
+func mine(int b) int {	// ERROR "undefined.*b"
+	return b + 2	// ERROR "undefined.*b"
+}
+
+func main() {
+	mine()
+	c = mine()	// ERROR "undefined.*c" "cannot assign to c"
+}