Fix long SNI lengths in test/handshake_helper.c

If the server_name extension is long enough to require two bytes to
hold the length of either field, the test suite would not decode
the length properly.  Using the PACKET_ APIs would have avoided this,
but it was desired to avoid using private APIs in this part of the
test suite, to keep ourselves honest.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4318)