Merge commit 'b1db3315'

* commit 'b1db3315':
  ffv1: Make sure at least one slice context is initialized

Conflicts:
	libavcodec/ffv1.c

This condition is not possible in ffmpegs ffv1 code and already
checked for by assert() for the case that some future changes make it possible.
Thus merge just for meta-data
Merged-by: Michael Niedermayer <michaelni@gmx.at>