lavfi/buffersrc: fix failed requests count.

The line was lost during the AVFrame transition.
It causes programs relying on the failed requests count,
especially ffmpeg, to add frames to the wrong input.

Fix trac ticket #2467.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi/buffersrc: fix failed requests count.

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