Fix a serious bug introduced into GIN in 8.4: now that MergeItemPointers()

Fix a serious bug introduced into GIN in 8.4: now that MergeItemPointers()
is supposed to remove duplicate heap TIDs, we have to be sure to reduce the
tuple size and posting-item count accordingly in addItemPointersToTuple().
Failing to do so resulted in the effective injection of garbage TIDs into the
index contents, ie, whatever happened to be in the memory palloc'd for the
new tuple.  I'm not sure that this fully explains the index corruption
reported by Tatsuo Ishii, but the test case I'm using no longer fails.