Remove a reachable assert from ssl3_write_bytes

A buggy application that call SSL_write with a different length after a
NBIO event could cause an OPENSSL_assert to be reached. The assert is not
actually necessary because there was an explicit check a little further
down that would catch this scenario. Therefore remove the assert an move
the check a little higher up.

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