smalloc: do not track external memory

The memory that was allocated outside of the `smalloc.cc` should not be
tracked using `AdjustAmountOfExternalAllocatedMemory`. There are no
potential issues except triggering V8's GC way too often.

In fact, `heap.js` is creating a buffer out of the pointer, and since it
doesn't know the size of the pointer - it just creates the maximum
possible `Buffer` instance with a no-op free callback and no hint.

PR-URL: https://github.com/iojs/io.js/pull/1375


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