sctp_crc32c: fix crc calculation in the NULL buffer case

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
test: enable the crc32c unitest
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
crc32c: expand unit test

Verify crc on a buffer range, and on a NULL buffer (with various
sizes).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>