xcbgrab: Support empty filename string correctly

Bug-Id: CID 1254664
CC: libav-stable@libav.org
Merge commit '2c3f29c4894ee50635b846f202296ad79a7c0d33'

* commit '2c3f29c4894ee50635b846f202296ad79a7c0d33':
  xcbgrab: Support empty filename string correctly

Conflicts:
	libavdevice/xcbgrab.c

See: 62eca2f827d441f52125191fd78c96b67a7ba30c
Merged-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_perspective: add av_assert0() to help gcc see that there are no uninitialized variables

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/flvdec: fix potential use of uninitialized variables

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>