tree data helpers BUGFIX store prefix for unprefixed value

Fixes #1843
diff --git a/src/tree_data_helpers.c b/src/tree_data_helpers.c
index 90d6164..b98ce77 100644
--- a/src/tree_data_helpers.c
+++ b/src/tree_data_helpers.c
@@ -972,6 +972,13 @@
             prefixes = *prefix_data_p;
         }
 
+        /* add current module for unprefixed values */
+        LY_ARRAY_NEW_GOTO(ctx, prefixes, val_pref, ret, cleanup);
+        *prefix_data_p = prefixes;
+
+        val_pref->prefix = NULL;
+        val_pref->mod = ((const struct lysp_module *)prefix_data)->mod;
+
         /* add all used prefixes */
         value_end = (char *)value + value_len;
         for (value_iter = value; value_iter; value_iter = value_next) {