Properly handle a partial block in OCB mode

If we have previously been passed a partial block in an "Update" call then
make sure we properly increment the output buffer when we use it.

Fixes #2273

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2275)
(cherry picked from commit 7c12c7b6)