Merge commit 'a7e79f6a43bdc7e3b164dd78b060f0d5575a0cf0' into release/2.4

* commit 'a7e79f6a43bdc7e3b164dd78b060f0d5575a0cf0':
  avfilter: check filter link validity

Conflicts:
	libavfilter/avfilter.c

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

libavfilter/avfilter.c

@@ -230,6 +230,11 @@ int avfilter_config_links(AVFilterContext *filter)
         AVFilterLink *inlink;
 
         if (!link) continue;
+        if (!link->src || !link->dst) {
+            av_log(filter, AV_LOG_ERROR,
+                   "Not all input and output are properly linked (%d).\n", i);
+            return AVERROR(EINVAL);
+        }
 
         inlink = link->src->nb_inputs ? link->src->inputs[0] : NULL;
         link->current_pts = AV_NOPTS_VALUE;