Fix LoadStreams missing frame bug (#4913)

ultralytics/data/loaders.py

@@ -127,8 +127,10 @@ class LoadStreams:
                 if not self.threads[i].is_alive() or cv2.waitKey(1) == ord('q'):  # q to quit
                     self.close()
                     raise StopIteration
-                LOGGER.warning(f'WARNING ⚠️ Waiting for stream {i}')
                 time.sleep(1 / min(self.fps))
+                x = self.imgs[i]
+                if not x:
+                    LOGGER.warning(f'WARNING ⚠️ Waiting for stream {i}')
 
             # Get and remove the first frame from imgs buffer
             if self.buffer: