Fix memory leak in GIN index scans.

The code had a query-lifespan memory leak when encountering GIN entries
that have posting lists (rather than posting trees, ie, there are a
relatively small number of heap tuples containing this index key value).
With a suitable data distribution this could add up to a lot of leakage.
Problem seems to have been introduced by commit 36a35c550, so back-patch
to 9.4.

Julien Rouhaud

src/backend/access/gin/ginget.c

@@ -278,6 +278,7 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
 			ipd = ginReadTuple(btree->ginstate, scanEntry->attnum, itup, &nipd);
 			tbm_add_tuples(scanEntry->matchBitmap, ipd, nipd, false);
 			scanEntry->predictNumberResult += GinGetNPosting(itup);
+			pfree(ipd);
 		}
 
 		/*