Summary: If we allow null characters in a regex, then a user could construct a string that ends with "/e\0" and surreptitiously execute an arbitrary piece of code. With this patch we fatal if a null is found anywhere before the end of the regex (determined by length()).
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.