Map transfer fails at 99% for certain players

[Skyaero42]

Prerequisites

• I have searched for similar issues and confirmed this is not a duplicate

Game Version

• Command & Conquer Generals
• Command & Conquer Generals: Zero Hour
• Other (please specify below)

Bug Description

When transferring map, the transfer may stall for certain players at 99% when transferring the .map file

Reproduction Steps

The bug only happens between certain players and is therefore hard to reproduce.
@LegionnaireG has this issue with certain players

Additional Context

No response

[Mauller]

Could be people with poor connections.

Might also be worth double checking the TSH client transfering a map to a retail client.

[Caball009]

I can reproduce this with multiple instances locally. It fails because the map I tried to transfer doesn't have the magic header bytes that are now a requirement: https://github.com/TheSuperHackers/GeneralsGameCode/pull/1819/changes#diff-04d95897fe75ae062970670cccf8cce7d9c4b1d84a8e626316a055b6b880d2b1R158

EDIT: I checked a random selection of 10 .map files and none has those specific header bytes at offset 0, but rather at offset 14.

At a quick glance this is needed to fix the issue:
if (dataSize < 4 || memcmp(data, "CkMp", 4) != 0)
->
if (dataSize < 18 || memcmp(data + 14, "CkMp", 4) != 0)