deps: fix v8 build on FreeBSD

clang++ on FreeBSD was blaming v8 for using invalid casts from nullptr:

    reinterpret_cast from 'nullptr_t' to '...' is not allowed

Replace casts with NULL, or NULL with 0 where applicable.

Fix #324
PR-URL: https://github.com/iojs/io.js/pull/332


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>