pyav decode leads memory leaks issue when 'duration' is none

Hi, I follow #541 changes to use pyav backend decode videos, and when i use 'pyav' for trainging, if 'duration' param is None, which is conditional at decoder.py Line 436, the memory will increase rapidly to the machine limit and training thread will stacked.

I try simply raise error when duration is none, just drop out these videos and training can works well, but i dont know why this happens and it will be better if i can use these videos for training again. Can someone helps? Thanks!