src: null env_ field from constructor

The env_ field in ArrayBufferAllocator needs to be null'd out since it
is used during initialization and checked prior to properly being set by
set_env().

Fixes: 74178a56 "buffer: construct Uint8Array in JS"
PR-URL: https://github.com/nodejs/node/pull/2913


Reviewed-By: Fedor Indutny <fedor@indutny.com>