mov: always check avio_read return value

If avio_read fails, the buffer can contain uninitialized data.

This fixes 'Conditional jump or move depends on uninitialised value(s)'
valgrind warnings, and addresses a few memleaks.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>