fix crash if SonarClient is stopped with no connection

Summary: calling SonarClient::instance()->stop() without a connection will try to deref a null client_

Reviewed By: jknoxville

Differential Revision: D9325527

fbshipit-source-id: b0a81d6eb9d1375b74cbf10466f947e8a2de3b98

xplat/Sonar/SonarWebSocketImpl.cpp

@@ -218,7 +218,9 @@ void SonarWebSocketImpl::reconnect() {
 }
 
 void SonarWebSocketImpl::stop() {
+  if (client_) {
     client_->disconnect();
+  }
   client_ = nullptr;
 }