Close generator already in dtor phase

In the added test case, the Closure ends up being freed before
the generator during GC.

This patch closes the generator (and thus releases

Close generator already in dtor phase

In the added test case, the Closure ends up being freed before
the generator during GC.

This patch closes the generator (and thus releases the held
closure / execute_data) already during dtor_obj, which will avoid
ordering issues in free_obj. dtor_obj is not always called, but
if it isn't, then we also won't run GC and will free_obj in
reverse construction order.

Fixes oss-fuzz #33947.

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