src: plug memory leaks

In a few places dynamic memory was passed to the Buffer::New() overload
that makes a copy of the input, not the one that takes ownership.

This commit is a band-aid to fix the memory leaks.  Longer term, we
should look into using C++11 move semantics more effectively.

Fixes: https://github.com/nodejs/node/issues/2308
PR-URL: https://github.com/nodejs/node/pull/2352


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>