GDT test server: stop server on dealloc only if running (#7253)

Author: maksymmalyhin

GoogleDataTransport/GDTCCTTests/Unit/TestServer/GDTCCTTestServer.m

@@ -49,7 +49,9 @@ - (instancetype)init {
 }
 
 - (void)dealloc {
-  [_server stop];
+  if (_server.isRunning) {
+    [_server stop];
+  }
 }
 
 - (void)start {