Fix crash as a result of MULTIBLOCK

The MULTIBLOCK code uses a "jumbo" sized write buffer which it allocates
and then frees later. Pipelining however introduced multiple pipelines. It
keeps track of how many pipelines are initialised using numwpipes.
Unfortunately the MULTIBLOCK code was not updating this when in deallocated
its buffers, leading to a buffer being marked as initialised but set to
NULL.

RT#4618

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