fs: account for buffer alloc failure in readFile

PR-URL: https://github.com/nodejs/node/pull/16219
Refs: https://github.com/nodejs/node/pull/15362


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>