variable referenced before assignment error

```
(tensorflow) dee@Dee:~/sandbox/medianflow$ python medianflow.py abhi.avi 
Traceback (most recent call last):
  File "medianflow.py", line 166, in <module>
    API("Median Flow Tracker", source).run()
  File "medianflow.py", line 133, in run
    frame = grabbed_frame.copy()
UnboundLocalError: local variable 'grabbed_frame' referenced before assignment

```