Add FPE protections against eta and rap calculations with (massless) null and z-axis vectors.

FPEs aren't a problem in normal circumstances, but in particular ATLAS runs with an FPE catching/auditing environment that treats them as undesirable. We are replying on the FP standard to generate +-inf eta and rap values for longitudinal vectors, but while numerically valid letting these happen automatically will flag the FP error state. Since we can see them coming, this patch should emit the same values but without the FPE. Hopefully the branch-prediction attributes will (eventually) offset any marginal inefficiency from the checking.