Fix a heap dumping deadlock.

Restructure the ctx initialization code such that the ctx isn't locked
across portions of the initialization code where allocation could occur.
Instead artificially inflate the cnt_merged.curobjs field, just as is
done elsewhere to avoid similar races to the one that would otherwise be
created by the reduction in locking scope.

This bug affected interval- and growth-triggered heap dumping, but not
manual heap dumping.