[Bug] Incorrect parsing of large handshake messages

[franziskuskiefer]

Parsing of large handshake messages is broken. The check for b.len() - 3 < l in check_lbytes3 fails.

To reproduce

BORINGSSL_ROOT=/path/to/boringssl ./bogo_shim/run.sh -test LargeMessage -include-disabled

See also #30

[karthikbhargavan]

I will debug this next week, thanks.