gistendscan() forgot to free so->giststate.

This oversight led to a massive memory leak --- upwards of 10KB per tuple
--- during creation-time verification of an exclusion constraint based on a
GIST index.  In most other scenarios it'd just be a leak of 10KB that would
be recovered at end of query, so not too significant; though perhaps the
leak would be noticeable in a situation where a GIST index was being used
in a nestloop inner indexscan.  In any case, it's a real leak of long
standing, so patch all supported branches.  Per report from Harald Fuchs.

src/backend/access/gist/gistscan.c

@@ -240,6 +240,7 @@ gistendscan(PG_FUNCTION_ARGS)
 	GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
 
 	freeGISTstate(so->giststate);
+	pfree(so->giststate);
 	MemoryContextDelete(so->queueCxt);
 	MemoryContextDelete(so->tempCxt);
 	pfree(so->tmpTreeItem);