MDEV-6329 - Buffer overrun in find_uniq_filename

Merged WebScaleSQL fix:
https://github.com/webscalesql/webscalesql-5.6/commit/bce9eddc1da87beab17c60d7eb7379841a3d45af

sql/log.cc

@@ -2244,7 +2244,7 @@ static int find_uniq_filename(char *name)
   file_info= dir_info->dir_entry;
   for (i= dir_info->number_off_files ; i-- ; file_info++)
   {
-    if (memcmp(file_info->name, start, length) == 0 &&
+    if (strncmp(file_info->name, start, length) == 0 &&
 	test_if_number(file_info->name+length, &number,0))
     {
       set_if_bigger(max_found,(ulong) number);