The extra memset meant pg_stat_reset_slru() always reset all the entries
even when reset of a single entry was requested, but the timestamp was
left uninitialized.

Reported-by: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYFe16pjZxQYaTn53mspyM7dgMPYL3DJLjjPw69GMCC2Ow%40mail.gmail.com