cifs: Fix oops in session setup code for null user mounts

For null user mounts, do not invoke string length function
during session setup.

Cc: <stable@kernel.org
Reported-and-Tested-by: Chris Clayton <chris2553@googlemail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>