hevc: do not dereference pointer before NULL check in verify_md5()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
hevc: store the VPS list as an AVBufferRef, just like the others *PS

Signed-off-by: Anton Khirnov <anton@khirnov.net>
hevc: set time_base when possible

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Replace all uses of avcodec_free_frame with av_frame_free().
lavc: deprecate avcodec_free_frame()

av_frame_free() should be used instead.
avconv: do not call avcodec_get_frame_defaults()

Strictly speaking it is not correct to call it on refcounted frames, in
any case it is unnecessary, since filtered_frame is always unreferenced
after poll_filter() returns.
lavc: call av_frame_unref() instead of avcodec_get_frame_defaults().

avcodec_get_frame_defaults() will be deprecated.
avplay: do not call avcodec_get_frame_defaults().

avcodec_decode_audio4() will reset the frame itself.
mpegvideo: remove an unneeded call to avcodec_get_frame_defaults().

ff_mpeg_unref_picture() already resets the frame.
mpegvideo: call av_frame_unref() instead of avcodec_get_frame_defaults().

This is a temporary workaround to allow deprecating
avcodec_get_frame_defaults(). The proper solution will be using a
properly allocated AVFrame in Picture.
lavf: remove an unneeded call to avcodec_get_frame_defaults().

avcodec_decode_*() resets the frame itself.
api-example: remove an unneeded call to avcodec_get_frame_defaults().

avcodec_decode_audio4() resets the frame itself.
h264: call av_frame_unref() instead of avcodec_get_frame_defaults().

This is a temporary workaround to allow deprecating
avcodec_get_frame_defaults(). The proper solution will be using a
properly allocated AVFrame in Picture.
lavc: deprecate avcodec_get_frame_defaults().

Also bump libavcodec micro and add an APIchanges entry saying that
av_frame_* should now be used instead of the lavc AVFrame functions.