Fix an old corner-case bug in set_config_option: push_old_value has to be

Fix an old corner-case bug in set_config_option: push_old_value has to be
called before, not after, calling the assign_hook if any.  This is because
push_old_value might fail (due to palloc out-of-memory), and in that case
there would be no stack entry to tell transaction abort to undo the GUC
assignment.  Of course the actual assignment to the GUC variable hasn't
happened yet --- but the assign_hook might have altered subsidiary state.
Without a stack entry we won't call it again to make it undo such actions.
So this is necessary to make the world safe for assign_hooks with side
effects.  Per a discussion a couple weeks ago with Magnus.

Back-patch to 8.0.  7.x did not have the problem because it did not have
allocatable stacks of GUC values.