avfilter/avfiltergraph: Add assert to write down in machine readable form what is assumed about sample rates in swap_samplerates_on_filter()

Fixes CID1397292

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5f2b360fc0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avfiltergraph.c

@@ -858,6 +858,8 @@ static void swap_samplerates_on_filter(AVFilterContext *filter)
         for (j = 0; j < outlink->in_samplerates->nb_formats; j++) {
             int diff = abs(sample_rate - outlink->in_samplerates->formats[j]);
 
+            av_assert0(diff < INT_MAX); // This would lead to the use of uninitialized best_diff but is only possible with invalid sample rates
+
             if (diff < best_diff) {
                 best_diff = diff;
                 best_idx  = j;