MuonGeoModelTestR4: Work around false-positive out-of-bounds array access warning

In the archtest build, we get a false-positive warning about an out-of-bounds array access involving a 3-element eigen vector. This should actually be addressed in future (post 3.4) versions of eigen (https://gitlab.com/libeigen/eigen/-/merge_requests/1820), but for now we can work around this by splitting the involved code off into a separate function.