Bug 1600705 - Avoid storing nullptr closed-over-bindings data for LazyScripts. r=jorendorff

The closed-over-binding data is generated for each scope with a nullptr
delimiter per scope. This is wasteful when there are no closed-over bindings.
This patch removes trailing nullptr entries and for leaf-functions may result
in the script-data allocation being avoided altogether.

The FullParseHandler::nextLazyClosedOverBinding() will return nullptr after
the end of the gcthings data so that the delazification process is otherwise
unchanged.

Differential Revision: https://phabricator.services.mozilla.com/D57885

--HG--
extra : moz-landing-system : lando