Lazily initialise the compression buffer

With read pipelining we use multiple SSL3_RECORD structures for reading.
There are SSL_MAX_PIPELINES (32) of them defined (typically not all of these
would be used). Each one has a 16k compression buffer allocated! This
results in a significant amount of memory being consumed which, most of the
time, is not needed.  This change swaps the allocation of the compression
buffer to be lazy so that it is only done immediately before it is actually
used.

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