test: fix memory leaks in malloc cctests

Make cctest valgrind-clean again by freeing heap-allocated memory.
Overlooked in commit ea94086a ("src: provide allocation + nullptr check
shortcuts.")

PR-URL: https://github.com/nodejs/node/pull/9667
Refs: https://github.com/nodejs/node/pull/8482


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>