add a missing free in subtyping (#31668)

(cherry picked from commit b08f120)

src/subtype.c

@@ -1138,6 +1138,7 @@ static int forall_exists_subtype(jl_value_t *x, jl_value_t *y, jl_stenv_t *e, in
         int set = e->Lunions.more;
         if (!sub || !set)
             break;
+        free(se.buf);
         save_env(e, &saved, &se);
         for (int i = set; i <= lastset; i++)
             statestack_set(&e->Lunions, i, 0);