Update bcrypt_pbkdf in other/other.c.

Patches error in stride calculations as per
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libutil/bcrypt_pbkdf.c.diff?r1=1.6;r2=1.7

reop wasn't affected by the bug, but still nice to keep the code
updated.
Merge pull request #3 from dchest/pbkdf

Update bcrypt_pbkdf in other/other.c.

Thanks!