context: Empty heap wrapper after removing tracer

This did not break before, but in SpiderMonkey 38 not doing this is more
likely to cause crashes because the pointer stored in the JS::Heap
wrapper is more likely to become invalid once it is no longer traced.
This would crash under SpiderMonkey 38 when executing JS::Heap's
destructor, so we should instead empty out the wrapper after removing the
tracer.

https://bugzilla.gnome.org/show_bug.cgi?id=776966