avfilter: do not crash on null link src/dst in avfilter_destroy()

Originally committed as revision 24719 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/avfilter.c

@@ -429,6 +429,7 @@ void avfilter_destroy(AVFilterContext *filter)
 
     for(i = 0; i < filter->input_count; i ++) {
         if(filter->inputs[i]) {
+            if (filter->inputs[i]->src)
             filter->inputs[i]->src->outputs[filter->inputs[i]->srcpad] = NULL;
             avfilter_formats_unref(&filter->inputs[i]->in_formats);
             avfilter_formats_unref(&filter->inputs[i]->out_formats);
@@ -437,6 +438,7 @@ void avfilter_destroy(AVFilterContext *filter)
     }
     for(i = 0; i < filter->output_count; i ++) {
         if(filter->outputs[i]) {
+            if (filter->outputs[i]->dst)
             filter->outputs[i]->dst->inputs[filter->outputs[i]->dstpad] = NULL;
             avfilter_formats_unref(&filter->outputs[i]->in_formats);
             avfilter_formats_unref(&filter->outputs[i]->out_formats);