avcodec/h264_refs: modify key frame detection heuristic to detect more cases

Fixes Ticket2968

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 5ac6b6028f17b64723884c9fa72cfcbd369a1ba2)

Conflicts:
	libavcodec/h264_refs.c
avcodec/h264: reduce noisiness of "mmco: unref short failure"

Do not consider it an error if we have no frames and should discard one.
This condition can easily happen when decoding is started from an I frame

Fixes Ticket2811

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 08a89761964bdd0a023eff6d37a1131fb7e1d7a0)

Conflicts:
	libavcodec/h264_refs.c