av_dict_set: fix potential memory leak with AV_DICT_DONT_OVERWRITE

av_dict_set leaks it key/value arguments if AV_DICT_DONT_OVERWRITE is
combined with AV_DICT_DONT_STRDUP_{KEY,VAL} and the key exists.

libavutil/dict.c

@@ -76,8 +76,11 @@ int av_dict_set(AVDictionary **pm, const char *key, const char *value,
         m = *pm = av_mallocz(sizeof(*m));
 
     if (tag) {
-        if (flags & AV_DICT_DONT_OVERWRITE)
+        if (flags & AV_DICT_DONT_OVERWRITE) {
+            if (flags & AV_DICT_DONT_STRDUP_KEY) av_free(key);
+            if (flags & AV_DICT_DONT_STRDUP_VAL) av_free(value);
             return 0;
+        }
         if (flags & AV_DICT_APPEND)
             oldval = tag->value;
         else