avcodec/hevcdec: decode at most one slice reporting being the first in the picture

Fixes deadlocks when decoding packets containing more than one of the aforementioned
slices when using frame threads.
Tested-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/hevcdec.c

@@ -2927,6 +2927,10 @@ static int decode_nal_unit(HEVCContext *s, const H2645NAL *nal)
         }
 
         if (s->sh.first_slice_in_pic_flag) {
+            if (s->ref) {
+                av_log(s->avctx, AV_LOG_ERROR, "Two slices reporting being the first in the same frame.\n");
+                goto fail;
+            }
             if (s->max_ra == INT_MAX) {
                 if (s->nal_unit_type == HEVC_NAL_CRA_NUT || IS_BLA(s)) {
                     s->max_ra = s->poc;