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.

Author: tglsfdc

src/backend/access/gist/gistscan.c

@@ -226,14 +226,17 @@ gistendscan(PG_FUNCTION_ARGS)
 		gistfreestack(so->stack);
 		gistfreestack(so->markstk);
 		if (so->giststate != NULL)
+		{
 			freeGISTstate(so->giststate);
+			pfree(so->giststate);
+		}
 		/* drop pins on buffers -- we aren't holding any locks */
 		if (BufferIsValid(so->curbuf))
 			ReleaseBuffer(so->curbuf);
 		if (BufferIsValid(so->markbuf))
 			ReleaseBuffer(so->markbuf);
 		MemoryContextDelete(so->tempCxt);
-		pfree(scan->opaque);
+		pfree(so);
 	}
 
 	PG_RETURN_VOID();