bpo-30074: Fix compile warnings of _PySlice_Unpack and convert missed

[serhiy-storchaka]

PySlice_GetIndicesEx in _ctypes.c.

[zhangyangyu]

There are still 2:

Objects/sliceobject.c: In function ‘slice_indices’:
Objects/sliceobject.c:297:25: warning: passing argument 1 of ‘_PySlice_Unpack’ from incompatible pointer type [-Wincompatible-pointer-types]
if (_PySlice_Unpack(self, &start, &stop, &step) < 0) {
^
Objects/sliceobject.c:134:1: note: expected ‘PyObject * {aka struct _object *}’ but argument is of type ‘PySliceObject * {aka struct *}’
_PySlice_Unpack(PyObject *_r,
^
Objects/sliceobject.c:289:41: warning: variable ‘slicelength’ set but not used [-Wunused-but-set-variable]
Py_ssize_t ilen, start, stop, step, slicelength;
^

[zhangyangyu]

The warnings are gone. :-)