cifs: check for bytes_remaining going to zero in CIFS_SessSetup

cifs: check for bytes_remaining going to zero in CIFS_SessSetup

commit fcda7f4578bbf9717444ca6da8a421d21489d078 upstream.

It's possible that when we go to decode the string area in the
SESSION_SETUP response, that bytes_remaining will be 0. Decrementing it at
that point will mean that it can go "negative" and wrap. Check for a
bytes_remaining value of 0, and don't try to decode the string area if
that's the case.

Reported-and-Acked-by: David Howells <dhowells@redhat.com>
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>