audio: protect recorder_->stop() from null pointer exception

Change-Id: Ibb7ed1c76dafc7368655b0751f94a4b0df6a531d

src/media/audio/opensl/opensllayer.cpp

@@ -397,9 +397,11 @@ OpenSLLayer::stopAudioCapture()
 
     {
         std::lock_guard<std::mutex> lck(recMtx);
+        if (recorder_) {
             recorder_->stop();
             recorder_.reset();
         }
+    }
     if (recThread.joinable()) {
         recCv.notify_all();
         recThread.join();