buffer: don't set zero fill for zero-length buffer

Instantiating a Buffer of length zero would set the kNoZeroFill flag to
true but never actually call ArrayBuffer::Allocator(). Which means the
flag was never set back to false. The result was that the next
allocation would unconditionally not be zero filled.

Add test to ensure Uint8Array's are zero-filled after creating a Buffer
of length zero. This test may falsely succeed, but will not falsely fail.

Fix: https://github.com/nodejs/node/issues/2930
PR-URL: https://github.com/nodejs/node/pull/2931


Reviewed-By: Rod Vagg <rod@vagg.org>