Protect uninit momentum in CaloPIDsChecker fix

avoid string comparison in track type selection in Calo/CaloMoniDst/src/CaloPIDsChecker.cpp in order to avoid a problem with gcc6.2 optimization, see details in !419 (comment 181451)

(added but then removed 'WIP' because it might be ok to merge this - I tested it on all platforms used by today's lhcb-head and lhcb-gaudi-head nightlies)