crypto: _afscrypto_strdup must copy trailing NUL

crypto: _afscrypto_strdup must copy trailing NUL

The _afscrypto_strdup helper function was failing to alloc enough
space for the string's trailing NUL, and to copy that NUL over.

Caught by coverity (#985580)

Change-Id: I61c9a96275a79682a9521846ab52615ea5bc061d
Reviewed-on: http://gerrit.openafs.org/9263
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>