avcodec/vp9: mark frame as finished on decode_tiles() failure

Fixes deadlock with framethreads
Fixes: Netflix_Aerial_1080p_60fps_8bit_420.y4m.vp9.noaltref.webm.ivf.s69372_r01-05_b6-.ivf
Fixes: Netflix_Aerial_1080p_60fps_10bit_420.y4m.vp9.noaltref.webm.ivf.s149104_r01-05_b6-.ivf
Fixes: ducks_take_off_444_720p50.y4m.vp9.webm.ivf.s107375_r01-05_b6-.ivf

Reported-by: James Zern <jzern@google.com>
Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
