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>
(cherry picked from commit 8aa8d12554)

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

libavcodec/hevc.c

@@ -108,7 +108,7 @@ static int pic_arrays_init(HEVCContext *s, const HEVCSPS *sps)
     if (!s->tab_ipm || !s->cbf_luma || !s->is_pcm)
         goto fail;
 
-    s->filter_slice_edges = av_malloc(ctb_count);
+    s->filter_slice_edges = av_mallocz(ctb_count);
     s->tab_slice_address  = av_malloc_array(pic_size_in_ctb,
                                       sizeof(*s->tab_slice_address));
     s->qp_y_tab           = av_malloc_array(pic_size_in_ctb,