Check for pacing queue emptiness before dequeuing

This shouldn't actually be reachable when the queue
is empty, but we can check anyway to be safe.

Ref: #2054

app/streaming/video/ffmpeg-renderers/pacer/pacer.cpp

@@ -249,7 +249,7 @@ void Pacer::handleVsync(int timeUntilNextVsyncMillis)
             return;
         }
 
-        if (m_Stopping) {
+        if (m_Stopping || m_PacingQueue.isEmpty()) {
             m_FrameQueueLock.unlock();
             return;
         }