Reset CurrentMemoryContext to TopMemoryContext at the beginning of error

cleanup, ie, as soon as we have caught the longjmp.  This ensures that
current context will be a valid context throughout error cleanup.  Before
it was possible that current context was pointing at a context that would
get deleted during cleanup, leaving any subsequent pallocs in deep
trouble.  I was able to provoke an Assert failure when compiled with
asserts + -DCLOBBER_FREED_MEMORY, if I did something that would cause
an error to be reported by the backend large-object code, because indeed
that code operates in a context that gets deleted partway through xact
abort --- and CurrentMemoryContext was still pointing at it!  Boo hiss.

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