Fix difference between end vertices of two composites i.e F.CHILD(1, F.VZ) - F.VZ
When migrating SLB (and BnoC) stripping lines to HLT2, there is cut on the end vertex difference between child and a parent (see Xib mother cut in stripping line). Here child is a a composite. The equivalent ThOr cut to invoke is F.CHILD(1, F.VZ) - F.VZ > something, but fails with:
opt/include/SelKernel/Utilities.h:300:16: error: member reference type 'const LHCb::Particle *const' is a pointer; did you mean to use '->'? return item.endVertex();
This MR fixes that.
To be tested with: DaVinci!635 (merged)