Buffer length underflow in LoginPacket causing unchecked exceptions to be thrown

Summary

### Impact `LoginPacket` uses `BinaryStream->getLInt()` to read the lengths of JSON payloads it wants to decode. Unfortunately, `BinaryStream->getLInt()` returns a signed integer, meaning that a malicious client can craft a packet with a large uint32 value for payload buffer size (which would be interpreted as a negative signed int32), causing `BinaryStream->get()` to throw an exception. In the context of PocketMine-MP, this leads to a server crash when the vulnerability is exploited. ### Patches e3fce7632b94e83fd6a518a87dcaf6a11681c4ac ### Workarounds This can be worked around by registering a custom `LoginPacket` implementation into `PacketPool` which overrides [this code](https://github.com/pmmp/BedrockProtocol/blob/47532c95ea37d5f0365b23f734d70d943ff95295/src/LoginPacket.php#L54) to patch it. ### For more information * Email us at [[email protected]](mailto:[email protected])