buffer: throw range error before truncating write

The check to determine whether `noAssert` was set to true and thus
whether RangeErrors should be thrown was happening after the write was
truncated to the available size of the buffer. These checks now occur in
the correct order.

Fixes: https://github.com/nodejs/node/issues/5587
PR-URL: https://github.com/nodejs/node/pull/5605


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