quaddtype: potential dereferencing bug in casts.cpp

[WarrenWeckesser]

In this code

numpy-user-dtypes/quaddtype/numpy_quaddtype/src/casts.cpp

Lines 34 to 43 in 2134a7d

	if (given_descrs[0]->backend != given_descrs[1]->backend)
	casting = NPY_UNSAFE_CASTING;
	Py_INCREF(given_descrs[0]);
	loop_descrs[0] = given_descrs[0];
	if (given_descrs[1] == NULL) {
	Py_INCREF(given_descrs[0]);
	loop_descrs[1] = given_descrs[0];
	}

given_descrs[1] is dereferenced in line 34 (given_descrs[1]->backend), so it is assumed that given_descrs[1] is not NULL. In line 40, however, there is a check for given_descrs[1] == NULL. So either the check in line 40 always fails, or there is a bug in line 34.