avcodec/hevc_ps: prevent stale pointer in malloc failure case

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 0fc2045d5f)

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

libavcodec/hevc_ps.c

@@ -959,6 +959,8 @@ int ff_hevc_decode_nal_sps(HEVCContext *s)
         if (s->sps_list[sps_id] && s->sps == (HEVCSPS*)s->sps_list[sps_id]->data) {
             av_buffer_unref(&s->current_sps);
             s->current_sps = av_buffer_ref(s->sps_list[sps_id]);
+            if (!s->current_sps)
+                s->sps = NULL;
         }
         av_buffer_unref(&s->sps_list[sps_id]);
         s->sps_list[sps_id] = sps_buf;