reflect: test, fix access to nil maps

Fixes #8010.

LGTM=bradfitz, khr
R=khr, bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/91450048

src/pkg/reflect/all_test.go

@@ -973,6 +973,28 @@ func TestMap(t *testing.T) {
 	}
 }
 
+func TestNilMap(t *testing.T) {
+	var m map[string]int
+	mv := ValueOf(m)
+	keys := mv.MapKeys()
+	if len(keys) != 0 {
+		t.Errorf(">0 keys for nil map: %v", keys)
+	}
+
+	// Check that value for missing key is zero.
+	x := mv.MapIndex(ValueOf("hello"))
+	if x.Kind() != Invalid {
+		t.Errorf("m.MapIndex(\"hello\") for nil map = %v, want Invalid Value", x)
+	}
+
+	// Check big value too.
+	var mbig map[string][10 << 20]byte
+	x = ValueOf(mbig).MapIndex(ValueOf("hello"))
+	if x.Kind() != Invalid {
+		t.Errorf("mbig.MapIndex(\"hello\") for nil map = %v, want Invalid Value", x)
+	}
+}
+
 func TestChan(t *testing.T) {
 	for loop := 0; loop < 2; loop++ {
 		var c chan int

src/pkg/runtime/hashmap.goc

@@ -908,11 +908,15 @@ func mapaccess2(t *MapType, h *Hmap, key *byte) (val *byte, pres bool) {
 
 #pragma textflag NOSPLIT
 func reflect·mapaccess(t *MapType, h *Hmap, key *byte) (val *byte) {
-	if(raceenabled && h != nil) {
+	if(h == nil)
+		val = nil;
+	else {
+		if(raceenabled) {
 			runtime·racereadpc(h, runtime·getcallerpc(&t), reflect·mapaccess);
 			runtime·racereadobjectpc(key, t->key, runtime·getcallerpc(&t), reflect·mapaccess);
 		}
 		val = hash_lookup(t, h, &key);
+	}
 }
 
 #pragma textflag NOSPLIT