Fix length check writing status request extension

Fix length check writing status request extension

The status request extension did not correctly check its length, meaning
that writing the extension could go 2 bytes beyond the buffer size. In
practice this makes little difference because, due to logic in buffer.c the
buffer is actually over allocated by approximately 5k!

Issue reported by Guido Vranken.

Reviewed-by: Rich Salz <rsalz@openssl.org>