evp/e_chacha20_poly1305.c: improve performance for short TLS records.

Inputs not longer than 64 bytes are processed ~10% faster, longer
lengths not divisble by 64, e.g. 255, up to ~20%. Unfortunately it's
impossible to measure with apps/speed.c, -aead benchmarks TLS-like
call sequence, but not exact. It took specially crafted code path...

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6597)
