_Stream.close(): Try to kill struct.pack() warnings when

_Stream.close():  Try to kill struct.pack() warnings when
writing the crc to file on the "PPC64 Debian trunk" buildbot
when running test_tarfile.

This is again a case where the native zlib crc is an unsigned
32-bit int, but the Python wrapper implicitly casts it to
signed C long, so that "the sign bit looks different" on
different platforms.