mlvdec: validate bits_per_coded_sample

A negative bits_per_coded_sample doesn't make sense.
If it is too large, the size calculation for av_get_packet overflows,
resulting in allocation of a too small buffer.

Also make sure width and height are sane.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
