reflect: don't panic on delete from nil map.

Fixes #8051

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/95560046

src/pkg/reflect/all_test.go

@@ -993,6 +993,9 @@ func TestNilMap(t *testing.T) {
 	if x.Kind() != Invalid {
 		t.Errorf("mbig.MapIndex(\"hello\") for nil map = %v, want Invalid Value", x)
 	}
+
+	// Test that deletes from a nil map succeed.
+	mv.SetMapIndex(ValueOf("hi"), Value{})
 }
 
 func TestChan(t *testing.T) {

src/pkg/runtime/hashmap.goc

@@ -990,7 +990,7 @@ func reflect·mapassign(t *MapType, h *Hmap, key *byte, val *byte) {
 #pragma textflag NOSPLIT
 func reflect·mapdelete(t *MapType, h *Hmap, key *byte) {
 	if(h == nil)
-		runtime·panicstring("delete from nil map");
+		return; // see bug 8051
 	if(raceenabled) {
 		runtime·racewritepc(h, runtime·getcallerpc(&t), reflect·mapdelete);
 		runtime·racereadobjectpc(key, t->key, runtime·getcallerpc(&t), reflect·mapdelete);