libyang BUGFIX NULL-pointer dereference

reported by coverity
diff --git a/src/printer_lyb.c b/src/printer_lyb.c
index dc03f61..ba2ad27 100644
--- a/src/printer_lyb.c
+++ b/src/printer_lyb.c
@@ -342,6 +342,7 @@
     LY_ARRAY_SIZE_TYPE u;
 
     if (!lybctx->subtrees) {
+        assert(lybctx->subtree_size == 0);
         u = 0;
     } else {
         u = LY_ARRAY_SIZE(lybctx->subtrees);