asfdec: only set asf_pkt->data_size after sanity checks

Otherwise invalid values are used unchecked in the next run.
This can cause NULL pointer dereferencing.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Alexandra Hájková <alexandra.khirnova@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>