Change free condition for tree.

src/interpreter.c

@@ -919,6 +919,8 @@ libab_result _interpreter_run(struct interpreter_state* state, libab_tree* tree,
             libab_ref_get(scope), tree->string_value, OPERATOR_POSTFIX);
         result = _interpreter_call_operator(state, to_call, into, scope,
                                             vec_index(&tree->children, 0));
+    } else {
+        libab_get_unit_value(state->ab, into);
     }
 
     if (needs_scope) {

src/tree.c

@@ -48,7 +48,8 @@ int _tree_foreach_free(void* data, va_list args) {
 }
 
 int _tree_needs_free(libab_tree* tree) {
-    return ((tree->variant == TREE_FUN && --tree->int_value) | (tree->variant != TREE_FUN));
+    return ((tree->variant == TREE_FUN && --(tree->int_value) == 0) |
+            (tree->variant != TREE_FUN));
 }
 
 void libab_tree_free_recursive(libab_tree* tree) {