Fixed in HEAD; please check.
a_numvals was added in September 2007 (slap.h 1.831 Fri Sep 21 06:43:56 2007); slapo-memberof.c was first committed in August 2007 (memberof.c 1.1 Fri Aug 24 00:46:58 2007), but developed about one year earlier. I'm surprised it survived so long without the a_numvals bit :)
Thanks, p.