smb3: remove static checker warning

Remove static checker warning pointed out by Dan Carpenter:

The patch feeaec621c09: "cifs: multichannel: move channel selection
above transport layer" from Apr 24, 2020, leads to the following
static checker warning:

        fs/cifs/smb2pdu.c:149 smb2_hdr_assemble()
        error: we previously assumed 'tcon->ses' could be null (see line 133)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
CC: Aurelien Aptel <aptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -85,7 +85,7 @@ static const int smb2_req_struct_sizes[NUMBER_OF_SMB2_COMMANDS] = {
 
 int smb3_encryption_required(const struct cifs_tcon *tcon)
 {
-	if (!tcon)
+	if (!tcon || !tcon->ses)
 		return 0;
 	if ((tcon->ses->session_flags & SMB2_SESSION_FLAG_ENCRYPT_DATA) ||
 	    (tcon->share_flags & SHI1005_FLAGS_ENCRYPT_DATA))