cmd/gc: fix nil pointer dereferences.

Fixes #5119.

R=golang-dev, dvyukov, dave, rsc
CC=golang-dev
https://golang.org/cl/7838050

src/cmd/gc/dcl.c

@@ -1339,6 +1339,8 @@ addmethod(Sym *sf, Type *t, int local, int nointerface)
 	f = methtype(pa, 1);
 	if(f == T) {
 		t = pa;
+		if(t == T) // rely on typecheck having complained before
+			return;
 		if(t != T) {
 			if(isptr[t->etype]) {
 				if(t->sym != S) {
@@ -1347,10 +1349,8 @@ addmethod(Sym *sf, Type *t, int local, int nointerface)
 				}
 				t = t->type;
 			}
-		}
-		if(t->broke) // rely on typecheck having complained before
-			return;
-		if(t != T) {
+			if(t->broke) // rely on typecheck having complained before
+				return;
 			if(t->sym == S) {
 				yyerror("invalid receiver type %T (%T is an unnamed type)", pa, t);
 				return;

src/cmd/gc/subr.c

@@ -714,6 +714,12 @@ methcmp(const void *va, const void *vb)
 	
 	a = *(Type**)va;
 	b = *(Type**)vb;
+	if(a->sym == S && b->sym == S)
+		return 0;
+	if(a->sym == S)
+		return -1;
+	if(b->sym == S)
+		return 1;
 	i = strcmp(a->sym->name, b->sym->name);
 	if(i != 0)
 		return i;
@@ -1393,7 +1399,7 @@ assignconv(Node *n, Type *t, char *context)
 	Node *r, *old;
 	char *why;
 	
-	if(n == N || n->type == T)
+	if(n == N || n->type == T || n->type->broke)
 		return n;
 
 	old = n;