avcodec/error_resilience: Fix mbskip_table==NULL codepath

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

libavcodec/error_resilience.c

@@ -1024,7 +1024,7 @@ void ff_er_frame_end(ERContext *s)
             const int mb_xy = s->mb_index2xy[i];
             int       error = s->error_status_table[mb_xy];
 
-            if (s->mbskip_table && !s->mbskip_table[mb_xy]) // FIXME partition specific
+            if (!s->mbskip_table || !s->mbskip_table[mb_xy]) // FIXME partition specific
                 distance++;
             if (error & (1 << error_type))
                 distance = 0;