Malicious clients can send invalid SFTP packets with payload size field set to
value 0x7ffffffc (2GB - 3B). This value incorrectly passes the validity check
on 32b platforms in the function sftp_decode_channel_data_to_packet().

This is caused by integer overflow in the packet length check. But the wrong
buffer bounds are not used for accessing beyond the buffer bounds as the
following call to `ssh_buffer_add_data()` will already fail to allocate buffer
of size > 256MB, leading to the self-DoS of the sftp server process.

This is exploitable for sftp servers running on on 32b platforms only.