cifs: smb2pdu: Fix potential NULL pointer dereference

tcon->ses is being dereferenced before it is null checked, hence
there is a potential null pointer dereference.

Fix this by moving the pointer dereference after tcon->ses has
been properly null checked.

Addresses-Coverity-ID: 1467426 ("Dereference before null check")
Fixes: 93012bf98416 ("cifs: add server->vals->header_preamble_size")
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Aurelien Aptel <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/smb2pdu.c

···
 build_qfs_info_req(struct kvec *iov, struct cifs_tcon *tcon, int level,
 		   int outbuf_len, u64 persistent_fid, u64 volatile_fid)
 {
-	struct TCP_Server_Info *server = tcon->ses->server;
+	struct TCP_Server_Info *server;
 	int rc;
 	struct smb2_query_info_req *req;
 	unsigned int total_len;
···
 
 	if ((tcon->ses == NULL) || (tcon->ses->server == NULL))
 		return -EIO;
+
+	server = tcon->ses->server;
 
 	rc = smb2_plain_req_init(SMB2_QUERY_INFO, tcon, (void **) &req,
 			     &total_len);