cifs: check rsp for NULL before dereferencing in SMB2_open

In SMB2_open there are several paths where the SendReceive2
call will return an error before it sets rsp_iov.iov_base
thus leaving iov_base uninitialized.

Thus we need to check rsp before we dereference it in
the call to get_rfc1002_length().

A report of this issue was previously reported in
http://www.spinics.net/lists/linux-cifs/msg12846.html

RH-bugzilla : 1476151

Version 2 :
* Lets properly initialize rsp_iov before we use it.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com&gt;.>
Signed-off-by: Steve French <smfrench@gmail.com>
Reported-by: Xiaoli Feng <xifeng@redhat.com>
CC: Stable <stable@vger.kernel.org>