Fix Floating Point Exception in counters.h

Nothing major as I guess one normally only runs over a single event during development.

fyi @sponce

• [2018-09-23 18:50] Validation started with lhcb-gaudi-merge#586
• [2018-09-24 00:09] Validation started with lhcb-lcg-dev3#658
• [2018-09-24 00:09] Validation started with lhcb-gaudi-head#2004
• [2018-09-24 00:10] Validation started with lhcb-dd4hep#6
• [2018-09-24 00:13] Validation started with lhcb-lcg-dev4#662
• [2018-09-24 00:14] Validation started with lhcb-tdr-test#297
• [2018-09-24 00:16] Validation started with lhcb-sanitizers#13
• [2018-09-24 09:35] Validation started with lhcb-tdr-test#298
• [2018-09-24 11:00] Validation started with lhcb-gaudi-head#2005

approved this merge request

changed milestone to %v30r4

approved this merge request

assigned to @leggett

approved this merge request

merged

mentioned in commit 69f3f1ff