avcodec/h264: Be more strict on rejecting pps_id changes

Fixes race condition
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 31cc9c04ca)

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

libavcodec/h264.c

@@ -1515,8 +1515,8 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size,
                 continue;
 
 again:
-            if (   !(avctx->active_thread_type & FF_THREAD_FRAME)
-                || nals_needed >= nal_index)
+            if (   (!(avctx->active_thread_type & FF_THREAD_FRAME) || nals_needed >= nal_index)
+                && !h->current_slice)
                 h->au_pps_id = -1;
             /* Ignore per frame NAL unit type during extradata
              * parsing. Decoding slices is not possible in codec init