ffmpeg: Clean up if filter initialisation failed to avoid a memleak.

Fixes ticket #3858.

Reviewed-by: Nicolas George
lavf/rtpdec_hevc: Fix compilation with -DDEBUG.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  lavf/rtpdec_hevc: Fix compilation with -DDEBUG.
  ffmpeg: Clean up if filter initialisation failed to avoid a memleak.

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