vorbis: use normal integer values for iterators

No need to use fixed-size integers.
vorbis: simplify the inner loop in setup_classifs
vorbis: fallback to normal division instead of crashing

The use of ff_inverse speeds up slightly arches + compilers that
do not provide a division faster than the whole machinery, such
as ppc32 + gcc4.7, but has operational limits.

Drop the always-enable assert and provide a fallback.
mpegvideo: unref old current_picture before allocating a new one

Allows decoding with only three frame buffers with frame reference
counting. Fixes VAAPI based decoding in vlc which uses only three
frame buffers for the mpegvideo-based codecs.
Merge commit '36f10380043c8b8c5cbf8c1add8695089cdaa341'

* commit '36f10380043c8b8c5cbf8c1add8695089cdaa341':
  vorbis: use normal integer values for iterators

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'ddf1b4a2f8a680126eb611428e4f47e6e5b8c6c0'

* commit 'ddf1b4a2f8a680126eb611428e4f47e6e5b8c6c0':
  vorbis: simplify the inner loop in setup_classifs

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'db347280eb3746d7b70888db97f1efe5f06f5624'

* commit 'db347280eb3746d7b70888db97f1efe5f06f5624':
  vorbis: fallback to normal division instead of crashing

Conflicts:
	libavcodec/vorbisdec.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  mpegvideo: unref old current_picture before allocating a new one

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avutil/hmac: silence pointer type warnings

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