ftp: No unbounded directory command re-use

OSS-Fuzz generated traffic containing a CWD command with a single very large
path argument (427kb) starting with ".___/` \x00\x00...", This is followed
by a large number of ftp replies with code 250. The directory logic in
ftp_reply() would match every incoming reply with the one pending CWD command,
triggering path buildup ending with something 120MB in size.

Protect from re-using a directory command by setting a flag in the
CmdArg record when it was consumed for the path traversal logic.

This doesn't prevent unbounded path build-up generally, but does prevent the
amplification of a single large command with very many small ftp_replies.
Re-using a pending path command seems like a bug as well.

scripts/base/protocols/ftp/utils-commands.zeek

@@ -10,6 +10,9 @@ export {
 		arg:  string &default="";
 		## Counter to track how many commands have been executed.
 		seq:  count &default=0;
+		## Flag indicating if the arg of this CmdArg has been used
+		## to update cwd of c$ftp.
+		cwd_consumed: bool &default=F;
 	};
 
 	## Structure for tracking pending commands in the event that the client