Update datasets.py

Video errors may occur.

Author: gmt710

utils/datasets.py

@@ -200,7 +200,7 @@ def __next__(self):
             # Read video
             self.mode = 'video'
             ret_val, img0 = self.cap.read()
-            if not ret_val:
+            while not ret_val:
                 self.count += 1
                 self.cap.release()
                 if self.count == self.nf:  # last video