src: provide allocation + nullptr check shortcuts

Provide shortcut `node::CheckedMalloc()` and friends that
replace `node::Malloc()` + `CHECK_NE(·, nullptr);` combinations
in a few places.

PR-URL: https://github.com/nodejs/node/pull/8482


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>