Buildfarm failures for hash indexes: buffer leaks

Hi all,

Looking at the buildfarm, serinus and moonjelly have just complained
about the failure of $subject:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=serinus&dt=2018-10-22%2006%3A34%3A02
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=moonjelly&dt=2018-10-20%2015%3A17%3A02

The first failure is unrelated to the involved commits, as they touched
completely different areas of the code:
INSERT INTO hash_split_heap SELECT a/2 FROM generate_series(1, 25000) a;
+ WARNING: buffer refcount leak: [6481] (rel=base/16384/32349, blockNum=156, flags=0x93800000, refcount=1 1)

And versions older than HEAD do not complain.

Any thoughts?
--
Michael