ctypes: fix CThunkObject_new()

* Initialize restype and flags fields to fix a crash when Python runs on a
  read-only file system
* Use Py_ssize_t type rather than int for the "i" iterator variable
* Reorder assignements to be able to more easily check if all fields are
  initialized

Issue #11048. Initial patch written by Marcin Bachry.
1 file changed