cifs: fix NULL pointer dereference in cifs_find_smb_ses

cifs: fix NULL pointer dereference in cifs_find_smb_ses

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: Paul Gortmaker <paul.gortmaker@windriver.com>