PrFTZoneHandler - Check array bounds before access

No index checking was applied before accessing the zone array.

In dd4hep builds this actually leads to out-of-bounds access due to some issues with the zone ID being larger than I presume was expected.

This will lead to exceptions being thrown in the dd4hep builds until that numbering is fixed, but this should be viewed as an improvement over out of bounds writing to random memory locations…

@lohenry FYI