Fix a number of places where brittle data structures or overly strong

Asserts would lead to a server core dump if an error occurred while
trying to abort a failed subtransaction (thereby leading to re-execution
of whatever parts of AbortSubTransaction had already run).  This of course
does not prevent such an error from creating an infinite loop, but at
least we don't make the situation worse.  Responds to an open item on
the subtransactions to-do list.

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