avconv: initialize hwaccel_pix_fmt

Fixes an assertion failure in vdpau_get_buffer() when -hwaccel auto is
used, but hwaccel init fails.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  avconv: initialize hwaccel_pix_fmt

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Revert "avcodec/hevc: calculate checksum only if AV_EF_EXPLODE is set"

This reverts commit 758b6d39f685a510f48ff9e4c05fffa859d23c42.

Requested-by: smarter
"now that http://git.videolan.org/?p=ffmpeg.git;a=commit;h=97de206b44a48da726807cc3e7b9448a8112760b
 has been merged, http://git.videolan.org/?p=ffmpeg.git;a=commit;h=758b6d39f685a510f48ff9e4c05fffa859d23c42
 could be reverted, this would reduce the delta with libav
 (and it makes sense)"