[CIFS] fix static checker warning

Dan Carpenter wrote:

The patch 7f420cee8bd6: "[CIFS] Charge at least one credit, if server
says that it supports multicredit" from Jun 23, 2013, leads to the
following Smatch complaint:

fs/cifs/smb2pdu.c:120 smb2_hdr_assemble()
         warn: variable dereferenced before check 'tcon->ses' (see line 115)

CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>