Do not overallocate for tmp.ciphers_raw

Do not overallocate for tmp.ciphers_raw

Well, not as much, at least.

Commit 07afdf3c3ac97af4f2b4eec22a97f7230f8227e0 changed things so
that for SSLv2 format ClientHellos we store the cipher list in the
TLS format, i.e., with two bytes per cipher, to be consistent with
historical behavior.

However, the space allocated for the array still performed the computation
with three bytes per cipher, a needless over-allocation (though a relatively
small one, all things considered).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2281)