Fixes for two memory exhaustion bugs:

* [Unbounded SSH field lengths can cause excessive memory allocation](https://github.com/connectbot/cbssh/security/advisories/GHSA-ch3q-cw5r-f4hg)
GHSA-ch3q-cw5r-f4hg
* [Excessive allocation and integer overflow in DER private-key parsing](https://github.com/connectbot/cbssh/security/advisories/GHSA-vc8p-8pxg-rfwg)
GHSA-vc8p-8pxg-rfwg