stat_item: tolerate NULL pointer argument in osmo_stat_item_group_free

Just like rate_ctr_group_free, osmo_stat_item_group_free should tolerate
it when the argument is NULL

Change-Id: I23323833e7268356a50c4fc6a19639c4ecd2a101
diff --git a/src/stat_item.c b/src/stat_item.c
index 1f398d5..804972b 100644
--- a/src/stat_item.c
+++ b/src/stat_item.c
@@ -224,6 +224,9 @@
 /*! Free the memory for the specified group of stat items */
 void osmo_stat_item_group_free(struct osmo_stat_item_group *grp)
 {
+	if (!grp)
+		return;
+
 	llist_del(&grp->list);
 	talloc_free(grp);
 }