schema compile FEATURE check of disabled leafref

Even if leafref is disabled by its if-features, all validity checks
must be performed.
diff --git a/src/tree_schema.c b/src/tree_schema.c
index b5bf8eb..6ecdf6c 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -1068,6 +1068,7 @@
 
     assert(!unres->ds_unres.xpath.count);
     assert(!unres->ds_unres.leafrefs.count);
+    assert(!unres->ds_unres.disabled_leafrefs.count);
     assert(!unres->ds_unres.dflts.count);
     assert(!unres->ds_unres.disabled.count);
 }