src: re-add `Realloc()` shrink after reading stream data

This would otherwise keep a lot of unused memory lying around,
and in particular add up to a page per chunk of memory overhead
for network reads, potentially opening a DoS vector if the resulting
`Buffer` objects are kept around indefinitely (e.g. stored in a list
and not concatenated until the socket finishes).

This fixes CVE-2018-7164.

Refs: https://github.com/nodejs-private/security/issues/186
Refs: https://github.com/nodejs/node/commit/7c4b09b24bbe7d6a8cbad256f47b30a101a909ea
PR-URL: https://github.com/nodejs-private/node-private/pull/129


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>