avfiltergraph: silence an uninitialized variable warning

The warning is:
libavfilter/avfiltergraph.c: In function ‘avfilter_graph_config’:
libavfilter/avfiltergraph.c:528:9: warning: ‘best_idx’ may be used uninitialized in this function [-Wuninitialized]
libavfilter/avfiltergraph.c:479:13: note: ‘best_idx’ was declared here

Initialize it to an invalid value and add an assert that it's properly
set later.

libavfilter/avfiltergraph.c

@@ -476,7 +476,7 @@ static void swap_channel_layouts_on_filter(AVFilterContext *filter)
 
     for (i = 0; i < filter->nb_outputs; i++) {
         AVFilterLink *outlink = filter->outputs[i];
-        int best_idx, best_score = INT_MIN, best_count_diff = INT_MAX;
+        int best_idx = -1, best_score = INT_MIN, best_count_diff = INT_MAX;
 
         if (outlink->type != AVMEDIA_TYPE_AUDIO ||
             outlink->in_channel_layouts->nb_channel_layouts < 2)
@@ -525,6 +525,7 @@ static void swap_channel_layouts_on_filter(AVFilterContext *filter)
                 best_count_diff = count_diff;
             }
         }
+        av_assert0(best_idx >= 0);
         FFSWAP(uint64_t, outlink->in_channel_layouts->channel_layouts[0],
                outlink->in_channel_layouts->channel_layouts[best_idx]);
     }