Not all applications (e.g. MPlayer) set block_align, and
when using a different demuxer it might not even be
easily available.
So fall back to selecting mode based on bit rate as before
if block_align has not useful value.
It can't be worse than failing to decode completely.

(cherry picked from commit 1d0d6305)

CC: libav-stable@libav.org
Signed-off-by: Reinhard Tartler <siretart@tauware.de>