smb: client: fix potential NULL deref in parse_dfs_referrals()

If server returned no data for FSCTL_DFS_GET_REFERRALS, @dfs_rsp will
remain NULL and then parse_dfs_referrals() will dereference it.

Fix this by returning -EIO when no output data is returned.

Besides, we can't fix it in SMB2_ioctl() as some FSCTLs are allowed to
return no data as per MS-SMB2 2.2.32.

Fixes: 9d49640a ("CIFS: implement get_dfs_refer for SMB2+")
Cc: stable@vger.kernel.org
Reported-by: Robert Morris <rtm@csail.mit.edu>
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2ops.c

@@ -2836,6 +2836,8 @@ smb2_get_dfs_refer(const unsigned int xid, struct cifs_ses *ses,
 		usleep_range(512, 2048);
 	} while (++retry_count < 5);
 
+	if (!rc && !dfs_rsp)
+		rc = -EIO;
 	if (rc) {
 		if (!is_retryable_error(rc) && rc != -ENOENT && rc != -EOPNOTSUPP)
 			cifs_tcon_dbg(VFS, "%s: ioctl error: rc=%d\n", __func__, rc);