runtime: deletion on nil maps is a no-op now

Fixes #4535.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6942044

src/pkg/runtime/hashmap.c

@@ -989,7 +989,7 @@ runtime·mapdelete(MapType *t, Hmap *h, ...)
 	byte *ak;
 
 	if(h == nil)
-		runtime·panicstring("deletion of entry in nil map");
+		return;
 
 	if(raceenabled)
 		runtime·racewritepc(h, runtime·getcallerpc(&t), runtime·mapdelete);

test/fixedbugs/issue4535.go

@@ -0,0 +1,12 @@
+// run
+
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+	var m map[int]int
+	delete(m, 0)
+}