avcodec/h264_slice: Clear top_borders on allocation

In case of bitstream errors the deblock filter and slices can access uninitialized
top_borders from previous slices which did not fill them as they stoped halfway due
to error or where entirely missing.

This also makes code using these tables deterministic in case of missing or damaged
slices

Found-by: Tyson Smith
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
