cmd/gc: more graceful handling of invalid fields in widstruct.

The protection against segfaults does not completely solve
crashes and breaks test/fixedbugs/bug365.go

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6615058

src/cmd/gc/align.c

@@ -54,8 +54,11 @@ 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;
+		if(f->type == T) {
+			// broken field, just skip it so that other valid fields
+			// get a width.
+			continue;
+		}
 		dowidth(f->type);
 		if(f->type->align > maxalign)
 			maxalign = f->type->align;

src/cmd/gc/go.h

@@ -146,7 +146,7 @@ struct	Type
 	uchar	copyany;
 	uchar	local;		// created in this file
 	uchar	deferwidth;
-	uchar	broke;
+	uchar	broke;  	// broken type definition.
 	uchar	isddd;		// TFIELD is ... argument
 	uchar	align;