dfa: Put our pointer check back.

The reimplementation by Libav does not prevent out of array
writes, even though it looks like it does at a quick glance.

No FFmpeg releases are affected by this

See: d1c95d2ce39560e251fdb14f4af91b04fd7b845c
     3623589edc7b1257bb45aa9e52c9631e133f22b6
     740ebe468c0567cac03ef7e6b4b9fd0253b97da2

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
