Fix Ostap test for ROOT 6.18

FYI @ibelyaev, this fixes 1 of the 2 Ostap tests failing in lhcb-lcg-dev4 because of the new ROOT 6.18, see https://lhcb-nightlies.cern.ch/logs/tests/nightly/lhcb-lcg-dev4/953/x86_64-centos7-gcc8-opt/Analysis/. So far I have not managed to sort out the segfault in test ostap_weighted. It's got to do with the clean-up of a data store and/or histogram. I tried various things but guess I do not know Ostap enough. Have you got an idea?