avcodec/error_resilience: check that er is supported before attempting to read the status of the previous slice

Fixes incorrectly set error_occured and improves speed

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

libavcodec/error_resilience.c

@@ -845,7 +845,7 @@ void ff_er_add_slice(ERContext *s, int startx, int starty,
     s->error_status_table[start_xy] |= VP_START;
 
     if (start_xy > 0 && !(s->avctx->active_thread_type & FF_THREAD_SLICE) &&
-        s->avctx->skip_top * s->mb_width < start_i) {
+        er_supported(s) && s->avctx->skip_top * s->mb_width < start_i) {
         int prev_status = s->error_status_table[s->mb_index2xy[start_i - 1]];
 
         prev_status &= ~ VP_START;