BUG: ufunc.at segfaults with non-trivial signature

Calling the method `at` of a ufunc with non-trivial signature can lead
to a segfault if all checks on the shape, number of arguments, etc, pass
and the actual computation is attempted, e.g. for the ufunc `matmul`.
Now, using `at` with a non-trivial signature will raise a TypeError
analogously the method `outer`.

numpy/core/src/umath/ufunc_object.c

@@ -5888,6 +5888,13 @@ ufunc_at(PyUFuncObject *ufunc, PyObject *args)
 
     NPY_BEGIN_THREADS_DEF;
 
+    if (ufunc->core_enabled) {
+        PyErr_Format(PyExc_TypeError,
+                     "method at is not allowed in ufunc with non-trivial"\
+                     " signature");
+        return NULL;
+    }
+
     if (ufunc->nin > 2) {
         PyErr_SetString(PyExc_ValueError,
             "Only unary and binary ufuncs supported at this time");