avcodec/h264_slice: Clear table pointers to avoid stale pointers

Might fix Ticket3889

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 547fce95858ef83f8c25ae347e3ae3b8ba437fd9)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>