Fix a memory leak in compression

The function RECORD_LAYER_clear() is supposed to clear the contents of the
RECORD_LAYER structure, but retain certain data such as buffers that are
allocated. Unfortunately one buffer (for compression) got missed and was
inadvertently being wiped, thus causing a memory leak.

In part this is due to the fact that RECORD_LAYER_clear() was reaching
inside SSL3_BUFFERs and SSL3_RECORDs, which it really shouldn't. So, I've
rewritten it to only clear the data it knows about, and to defer clearing
of SSL3_RECORD and SSL3_BUFFER structures to SSL_RECORD_clear() and the
new function SSL3_BUFFER_clear().

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