longs where the top bit is set.  First, change the masks so that they
are `L' longs, otherwise the sign bits will get propagated to the
result.  Next, do not coerce to int before sending to pack_uint()
otherwise Python will generate an OverflowError.  Here is a test
program that fails without the patch, but now succeeds:

import xdrlib

addr = (132, 151, 1, 71)
uint = 0L
for a in addr:
    uint = (uint << 8) | a

ulong64 = uint << 32

p = xdrlib.Packer()
p.pack_uhyper(ulong64)
buf = p.get_buffer()
u = xdrlib.Unpacker(buf)
ulong64prime = u.unpack_uhyper()

if ulong64 == ulong64prime:
    print 'okay'
else:
    print 'bogus'

print ulong64, ulong64prime