libdm: make dm_stats_destroy(NULL) a no-op

Test for NULL in dm_stats_destroy() and return immediately if
the struct dm_stats pointer is NULL (similar to free(NULL)).

This simplifies cleanup code which otherwise needs to:

  out:
    if (dms)
        dm_stats_destroy(dms);
    return;

libdm/libdm-stats.c

@@ -2387,6 +2387,9 @@ bad:
  */
 void dm_stats_destroy(struct dm_stats *dms)
 {
+	if (!dms)
+		return;
+
 	_stats_regions_destroy(dms);
 	_stats_groups_destroy(dms);
 	_stats_clear_binding(dms);