Avoid memory leakage when a series of subtransactions invoke AFTER triggers

that are fired at end-of-statement (as is the normal case for foreign keys,
for example).  In this situation the per-subxact deferred trigger context
is always empty when subtransaction exit is reached; so we could free it,
but were not doing so, leading to an intratransaction leak of 8K or more
per subtransaction.  Per off-list example from Viatcheslav Kalinin
subsequent to bug #3418 (his original bug report omitted a foreign key
constraint needed to cause this leak).

Back-patch to 8.2; prior versions were not using per-subxact contexts
for deferred triggers, so did not have this leak.