alsa-audio-dec: explicitly cast the delay to a signed int64

Otherwise the expression will be evaluated as unsigned, which will break
when the result should be negative.
CC:libav-stable@libav.org
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  alsa-audio-dec: explicitly cast the delay to a signed int64

Conflicts:
	libavdevice/alsa-audio-dec.c

no change as the buggy code has been replaced in ffmpeg

Merged-by: Michael Niedermayer <michaelni@gmx.at>
vp9_parser: handle zero-sized packets.

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