avcodec/h264_slice: Limit max_contexts when slice_context_count is initialized

Fixes out of array access
Fixes: 1430e9c43fae47a24c179c7c54f94918/signal_sigsegv_421427_2049_f2192b6829ab6e0eefcb035329c03c60.264

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_slice.c

@@ -1097,6 +1097,7 @@ static int h264_slice_header_init(H264Context *h)
         nb_slices = max_slices;
     }
     h->slice_context_count = nb_slices;
+    h->max_contexts = FFMIN(h->max_contexts, nb_slices);
 
     if (!HAVE_THREADS || !(h->avctx->active_thread_type & FF_THREAD_SLICE)) {
         ret = ff_h264_slice_context_init(h, &h->slice_ctx[0]);