* Defend against possible integer overflows in comp_method_zlib_decomp.
* Defend against writing beyond the end of the payload in _libssh2_transport_read().
* Sanitize padding_length - _libssh2_transport_read(). https://libssh2.org/CVE-2019-3861.html
This prevents an underflow resulting in a potential out-of-bounds read if a server sends a too-large padding_length, possibly with malicious intent.
* Prevent zero-byte allocation in sftp_packet_read() which could lead to an out-of-bounds read. https://libssh2.org/CVE-2019-3858.html
* Check the length of data passed to sftp_packet_add() to prevent out-of-bounds reads.
* Add a required_size parameter to sftp_packet_require et. al. to require callers of these functions to handle packets that are too short. https://libssh2.org/CVE-2019-3860.html
* Additional length checks to prevent out-of-bounds reads and writes in _libssh2_packet_add(). https://libssh2.org/CVE-2019-3862.html