buffer: SlowBuffer only accept valid numeric values

Fixes a regression that appeared with the new Buffer implementation in v3.
Without this change, calling the SlowBuffer constructor with something else
than a number would abort on the C++ side. This makes sure that the length
argument is coerced to number or is 0.

Fixes: https://github.com/nodejs/node/issues/2634
PR-URL: https://github.com/nodejs/node/pull/2635


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>