avcodec/hevc: clear filter_slice_edges() on allocation

This avoids use of uninitialized memory
Fixes: asan_static-oob_17aa046_582_cov_212287884_DBLK_G_VIXS_1.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>