schema BUGFIX resolving prefix in schemas

incorrect check could cause not resolving default prefix.
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index d807838..d6c92da 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -1026,7 +1026,7 @@
     struct lys_module *m = NULL;
     LY_ARRAY_COUNT_TYPE u;
 
-    if (!prefix || !ly_strncmp(mod->prefix, prefix, len)) {
+    if (!len || !ly_strncmp(mod->prefix, prefix, len)) {
         /* it is the prefix of the module itself */
         m = (struct lys_module *)mod;
     }