9.4 failure on skink in _bt_newroot/XLogCheckBuffer

The valgrind animal just reported a large object related failure on 9.4:

http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=skink&dt=2016-05-19%2006%3A23%3A05

==9952== VALGRINDERROR-BEGIN
==9952== Conditional jump or move depends on uninitialised value(s)
==9952==    at 0x4DC6D3: XLogCheckBuffer (xlog.c:2077)
==9952==    by 0x4E5E52: XLogInsert (xlog.c:956)
==9952==    by 0x4ACB10: _bt_newroot (nbtinsert.c:2123)
==9952==    by 0x4ACEFF: _bt_insert_parent (nbtinsert.c:1727)
==9952==    by 0x4AD4B7: _bt_insertonpg (nbtinsert.c:776)
==9952==    by 0x4AE56F: _bt_doinsert (nbtinsert.c:191)
==9952==    by 0x4B3409: btinsert (nbtree.c:251)
==9952==    by 0x7A87E3: FunctionCall6Coll (fmgr.c:1437)
==9952==    by 0x4A8D36: index_insert (indexam.c:226)
==9952==    by 0x4FC62C: CatalogIndexInsert (indexing.c:136)
==9952==    by 0x6A7210: inv_write (inv_api.c:723)
==9952==    by 0x5E2985: lo_write (be-fsstubs.c:223)
==9952==  Uninitialised value was created by a stack allocation
==9952==    at 0x4AC481: _bt_newroot (nbtinsert.c:1989)
==9952==
==9952== VALGRINDERROR-END

I've not analyzed the problem beyond noticing that
xlog.c:2077        if (rdata->buffer_std)
which suggests an actual bug.

Regards,

Andres