RETURN_FALSE instead of return in some methods

There appears to be a codepath in luasandbox_load_helper and in
luasandbox_call_helper where PHP's return_value parameter might never
get set. Other codepaths in these functions call RETURN_FALSE, so let's
do that for these too.

Change-Id: I76609c044ffe8e1d3969945b9a4aefcf0d13318a