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.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/formats.c

@@ -45,6 +45,9 @@ AVFilterFormats *avfilter_merge_formats(AVFilterFormats *a, AVFilterFormats *b)
     AVFilterFormats *ret;
     unsigned i, j, k = 0, m_count;
 
+    if (a == b)
+        return a;
+
     ret = av_mallocz(sizeof(*ret));
 
     /* merge list of formats */