schema tree BUGFIX add missing string keyword

Refs #1573
diff --git a/src/tree_schema.c b/src/tree_schema.c
index 2881cb2..ac876d1 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -130,7 +130,11 @@
 API const char *
 ly_stmt2str(enum ly_stmt stmt)
 {
-    return stmt_attr_info[stmt].name;
+    if (stmt == LY_STMT_EXTENSION_INSTANCE) {
+        return "extension instance";
+    } else {
+        return stmt_attr_info[stmt].name;
+    }
 }
 
 const char * const ly_devmod_list[] = {