PrForwardTracking - Fix ubsan error due to null pointer dereferencing
Addresses a number of undefined behaviour errors at
https://lhcb-nightlies.cern.ch/nightly/lhcb-sanitizers/build/92/
/workspace/build/REC/REC_HEAD/Pr/PrAlgorithms/src/PrForwardTracking.cpp:326:47: runtime error: reference binding to null pointer of type 'struct ISequencerTimerTool'
#0 0x7f4b7ec7b8a8 in PrForwardTracking::operator()(PrFTHitHandler<PrHit> const&, Pr::Selection<LHCb::Event::v2::Track, unsigned short> const&) const /workspace/build/REC/REC_HEAD/Pr/PrAlgorithms/src/PrForwardTracking.cpp:326