vp9: fix mvref finding to adhere to bug in libvpx.

Fixes a particular youtube video that I unfortunately can't share.
avformat/thp: do not set sample_rate for a video codec

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/thp: check fps value during probing

Fixes probetest failure

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