Bug#853559: mozjs24: ftbfs with GCC-7

https://github.com/uhulinux/ub-dev/commit/22ddd3bb42c4b8fb43f7c7263acf86900ea97b2c -- R.

Bug#853559: mozjs24: ftbfs with GCC-7

Simply replace '\0' with NULL in js/src/shell/jsoptparse.cpp:256. -- R.