it suffices to check for PY_SSIZE_T_MAX overflow (#22643)

Objects/unicodeobject.c

@@ -9484,12 +9484,11 @@ case_operation(PyObject *self,
     kind = PyUnicode_KIND(self);
     data = PyUnicode_DATA(self);
     length = PyUnicode_GET_LENGTH(self);
-    if (length > PY_SSIZE_T_MAX / 3 ||
-        length > PY_SIZE_MAX / (3 * sizeof(Py_UCS4))) {
+    if (length > PY_SSIZE_T_MAX / (3 * sizeof(Py_UCS4))) {
         PyErr_SetString(PyExc_OverflowError, "string is too long");
         return NULL;
     }
-    tmp = PyMem_MALLOC(sizeof(Py_UCS4) * 3 * (size_t)length);
+    tmp = PyMem_MALLOC(sizeof(Py_UCS4) * 3 * length);
     if (tmp == NULL)
         return PyErr_NoMemory();
     newlength = perform(kind, data, length, tmp, &maxchar);