Issue #18408: Different fixes in _elementtree.c to handle correctly MemoryError

Issue #18408: Different fixes in _elementtree.c to handle correctly MemoryError

 * create_new_element() initializes all attributes before handling errors,
   to fix a crash in the destructor
* create_new_element() calls PyObject_GC_Del() on error, instead of
  PyObject_Del(), because the object was created by PyObject_GC_New()
* subelement() now handles create_new_element() failure
* element_getattro() now handles element_get_text() failure
* makeuniversal() now handles PyBytes_FromStringAndSize() failure