channelstorage_cpp_map_name_id: Fix callback returning non-matching channels.

When the callback() API was invoked but no channel passed the test, callback
would return the last channel tested instead of NULL.  It now correctly
returns NULL when no channel matches.

Resolves: #1288

main/channelstorage_cpp_map_name_id.cc

@@ -151,12 +151,13 @@ static struct ast_channel *callback(struct ast_channelstorage_instance *driver,
 		chan = it->second;
 		if (cb_fn(chan, arg, data, ao2_flags) == (CMP_MATCH | CMP_STOP)) {
 			ao2_bump(chan);
-			break;
+			unlock(driver);
+			return chan;
 		}
 	}
 	unlock(driver);
 
-	return chan;
+	return NULL;
 }
 
 enum cpp_map_iterator_type {