bpo-30074: Fix compile warnings of _PySlice_Unpack and convert missed (#1154)

PySlice_GetIndicesEx in _ctypes.c.
diff --git a/Objects/sliceobject.c b/Objects/sliceobject.c
index 3ac3811..8f17fca 100644
--- a/Objects/sliceobject.c
+++ b/Objects/sliceobject.c
@@ -286,7 +286,7 @@
 static PyObject*
 slice_indices(PySliceObject* self, PyObject* len)
 {
-    Py_ssize_t ilen, start, stop, step, slicelength;
+    Py_ssize_t ilen, start, stop, step;
 
     ilen = PyNumber_AsSsize_t(len, PyExc_OverflowError);
 
@@ -294,10 +294,10 @@
         return NULL;
     }
 
-    if (_PySlice_Unpack(self, &start, &stop, &step) < 0) {
+    if (_PySlice_Unpack((PyObject *)self, &start, &stop, &step) < 0) {
         return NULL;
     }
-    slicelength = _PySlice_AdjustIndices(ilen, &start, &stop, step);
+    _PySlice_AdjustIndices(ilen, &start, &stop, step);
 
     return Py_BuildValue("(nnn)", start, stop, step);
 }