src: fix uninitialized memory dereference

The elements of the heap-allocated TaskQueue::ring_ array in
src/node_v8_platform.cc were compared against without being
initialized first.

Fixes node-forward/node#33.

PR-URL: https://github.com/node-forward/node/pull/34


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