lavfi: avfilter_merge_formats: handle case where inputs are same

This fixes a double-free crash if lists are the same due to the two
merge_ref() calls at the end of the (useless) merging that happens.

libavfilter/formats.c

@@ -44,6 +44,8 @@ AVFilterFormats *avfilter_merge_formats(AVFilterFormats *a, AVFilterFormats *b)
     AVFilterFormats *ret;
     unsigned i, j, k = 0;
 
+    if (a == b) return a;
+
     ret = av_mallocz(sizeof(AVFilterFormats));
 
     /* merge list of formats */