[cifs] fix null pointer check

Dan Carpenter pointed out an inconsistent null pointer check
in smb2_hdr_assemble that was pointed out by static checker.
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Sachin Prabhu <sprabhu@redhat.com>
CC: Dan Carpenter <dan.carpenter@oracle.com>w

fs/cifs/smb2pdu.c

@@ -110,7 +110,7 @@ smb2_hdr_assemble(struct smb2_hdr *hdr, __le16 smb2_cmd /* command */ ,
 
 	/* GLOBAL_CAP_LARGE_MTU will only be set if dialect > SMB2.02 */
 	/* See sections 2.2.4 and 3.2.4.1.5 of MS-SMB2 */
-	if ((tcon->ses) &&
+	if ((tcon->ses) && (tcon->ses->server) &&
 	    (tcon->ses->server->capabilities & SMB2_GLOBAL_CAP_LARGE_MTU))
 		hdr->CreditCharge = cpu_to_le16(1);
 	/* else CreditCharge MBZ */