inititilize counters (fix valgrind error)

Valgrind errors suggest that the std::array<std::atomic_int,15> used as counters in MuonSegmentTagTool are uninitialized. Reading through cppreference.com I understand that atomic_int and alike are usually zero-initialized but not when they are part of an std::array. Explicitly initializing them fixes the valgrind complains and the numbers reported in finalize are more reasonable.

Tagging @oda