ffmdec: limit the backward seek to the last resync position

If resyncing leads to the same position as previously, it will again
lead to a resync attempt, resulting in an infinite loop.

Thus don't seek back beyond the last syncpoint.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
