Revert "mpegvideo: remove abort() in ff_find_unused_picture()"

This reverts commit 4f820131fa9fbb0a64d7cc469fa471905fc91944.

It is better to abort() than to have remotly exploitable arbitrary code
execution bugs. Even more so that this abort has never been triggered
by any input people threw at it.
If after more extensive testing its removial is found safe we can remove
the abort() later.
