lib,src: reset zero fill flag on exception

Exceptions thrown from the Uint8Array constructor would leave it
disabled.

Regression introduced in commit 27e84ddd ("lib,src: clean up
ArrayBufferAllocator") from two days ago.  A follow-up commit
will add a regression test.

PR-URL: https://github.com/nodejs/node/pull/7093


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>