src: fix backport for SIGINT crash fix on FreeBSD

61fe1fe backported
b64983d 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 nodejs#9326.

Author: Julien Gilli

src/node.cc

@@ -2790,7 +2790,7 @@ static void SignalExit(int signo) {
   struct sigaction sa;
   memset(&sa, 0, sizeof(sa));
   sa.sa_handler = SIG_DFL;
-  CHECK_EQ(sigaction(signo, &sa, nullptr), 0);
+  CHECK_EQ(sigaction(signo, &sa, NULL), 0);
 #endif
   raise(signo);
 }