allow range on nil maps

R=ken
OCL=26663
CL=26663

src/runtime/hashmap.c

@@ -870,6 +870,10 @@ sys·mapassign2(Hmap *h, ...)
 void
 sys·mapiterinit(Hmap *h, struct hash_iter *it)
 {
+	if(h == nil) {
+		it->data = nil;
+		return;
+	}
 	hash_iter_init(h, it);
 	it->data = hash_next(it);
 	if(debug) {

test/map.go

@@ -487,4 +487,10 @@ func main() {
 			fmt.Printf("update mipM[%d][%d] = %i\n", i, i, mipM[i][i]);
 		}
 	}
+	
+	// test range on nil map
+	var mnil map[string] int;
+	for x, y := range mnil {
+		panic("range mnil");
+	}
 }