Avoid inplace modifyingimgs in LoadStreams (#5850)

When OpenCV retrieving image fail, original code would modify source images **inplace**, which may result in plotting bounding boxes on a black image. That is, before inference, source image `im0s[i]` is OK, but after inference before `Process predictions`,  `im0s[i]` may have been changed.

utils/datasets.py

@@ -337,7 +337,7 @@ class LoadStreams:
                     self.imgs[i] = im
                 else:
                     LOGGER.warning('WARNING: Video stream unresponsive, please check your IP camera connection.')
-                    self.imgs[i] *= 0
+                    self.imgs[i] = np.zeros_like(self.imgs[i])
                     cap.open(stream)  # re-open stream if signal was lost
             time.sleep(1 / self.fps[i])  # wait time