src: fix backport for SIGINT crash fix on FreeBSD

61fe1fe2 backported
b64983d7

 from io.js, but failed to
change nullptr to NULL, which lead to a build break on FreeBSD since the
current build system doesn't enable support for C++11.

This change replaces nullptr by NULL, and has been tested on
FreeBSD 10.1-RELEASE-p8.

Fixes #9326.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/joyent/node/pull/14819