Untested code for 64-bit platforms. range_length() is declared as int

but returns r->len which is a long.  This doesn't even cause a warning
on 32-bit platforms, but can return bogus values on 64-bit platforms
(and should cause a compiler warning).  Fix this by inserting a range
check when LONG_MAX != INT_MAX, and adding an explicit cast to (int)
when the test passes.  When r->len is out of range, PySequence_Size()
and hence len() will report an error (but an iterator will still
work).