runtime: Delete from a nil map is now a no-op.

From-SVN: r194462

gcc/testsuite/go.test/test/nil.go

@@ -151,9 +151,8 @@ func maptest() {
 	shouldPanic(func() {
 		m[2] = 3
 	})
-	shouldPanic(func() {
-		delete(m, 2)
-	})
+	// can delete (non-existent) entries
+	delete(m, 2)
 }
 
 // nil slice

libgo/runtime/go-map-delete.c

@@ -27,7 +27,7 @@ __go_map_delete (struct __go_map *map, const void *key)
   void **pentry;
 
   if (map == NULL)
-    runtime_panicstring ("deletion of entry in nil map");
+    return;
 
   descriptor = map->__descriptor;