cext: stop preprocessing a partial function call

MSVC++ 14 yelled:

    mercurial/cext/revlog.c(1913): fatal error C1057: unexpected end of file in
                                   macro expansion

At this point, the C extensions build (with warnings), and it dies in win32.py
because the `_fields_` strings in the ctypes classes are being converted to
bytes by the source translator.

mercurial/cext/revlog.c

@@ -1911,10 +1911,11 b' static int index_slice_del(indexObject *'
 /* Argument changed from PySliceObject* to PyObject* in Python 3. */
 #ifdef IS_PY3K
 	if (PySlice_GetIndicesEx(item, length,
+				 &start, &stop, &step, &slicelength) < 0)
 #else
 	if (PySlice_GetIndicesEx((PySliceObject*)item, length,
+				 &start, &stop, &step, &slicelength) < 0)
 #endif
-				 &start, &stop, &step, &slicelength) < 0)
 		return -1;
 
 	if (slicelength <= 0)