cifs: check for NULL session password

BugLink: http://bugs.launchpad.net/bugs/823296

commit 24e6cf92fde1f140d8eb0bf7cd24c2c78149b6b2 upstream.

It's possible for a cifsSesInfo struct to have a NULL password, so we
need to check for that prior to running strncmp on it.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>