Fix a compression bug

do_ssl3_write() was crashing when compression was enabled. We calculate
the maximum length that a record will be after compression and reserve
those bytes in the WPACKET. Unfortunately we were adding the maximum
compression overhead onto the wrong variable resulting in a corrupted
record.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2814)