[SMB3] Missing null tcon check

Pointed out by Dan Carpenter via smatch code analysis tool

CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <steve.french@primarydata.com>

fs/cifs/smb2pdu.c

@@ -922,7 +922,7 @@ SMB2_tcon(const unsigned int xid, struct cifs_ses *ses, const char *tree,
 	if (tcon && tcon->bad_network_name)
 		return -ENOENT;
 
-	if ((tcon->seal) &&
+	if ((tcon && tcon->seal) &&
 	    ((ses->server->capabilities & SMB2_GLOBAL_CAP_ENCRYPTION) == 0)) {
 		cifs_dbg(VFS, "encryption requested but no server support");
 		return -EOPNOTSUPP;