git-cvsserver: protect against NULL in crypt(3)

git-cvsserver: protect against NULL in crypt(3)

Some versions of crypt(3) will return NULL when passed an unsupported
hash type (ex: OpenBSD with DES), so check for undef instead of using
it directly.

Also use this to probe the system and select a better hash function in
the tests, so it can pass successfully.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
[jc: <CAPUEspjqD5zy8TLuFA96usU7FYi=0wF84y7NgOVFqegtxL9zbw@mail.gmail.com>]
Signed-off-by: Junio C Hamano <gitster@pobox.com>