avcodec/h264: Fail for invalid mixed IDR / non IDR frames in slice threading mode

Fixes Ticket4408
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit fc58d5c4)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -1551,8 +1551,14 @@ again:
                     ret = -1;
                     goto end;
                 }
-                if(!idr_cleared)
+                if(!idr_cleared) {
+                    if (h->current_slice && (avctx->active_thread_type & FF_THREAD_SLICE)) {
+                        av_log(h, AV_LOG_ERROR, "invalid mixed IDR / non IDR frames cannot be decoded in slice multithreading mode\n");
+                        ret = AVERROR_INVALIDDATA;
+                        goto end;
+                    }
                     idr(h); // FIXME ensure we don't lose some frames if there is reordering
+                }
                 idr_cleared = 1;
                 h->has_recovery_point = 1;
             case NAL_SLICE: