cifs: fix NULL pointer dereference in cifs_find_smb_ses

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

commit fc87a40677bbe0937e2ff0642c7e83c9a4813f3d upstream.

cifs_find_smb_ses assumes that the vol->password field is a valid
pointer, but that's only the case if a password was passed in via
the options string. It's possible that one won't be if there is
no mount helper on the box.

Reported-by: diabel <gacek-2004@wp.pl>
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>