Set `cluster_z` to zero when outside gas volume.
Slightly wrong Fatras geometry results in z-value of hits to
be in some cases largely outside the TRT gas volume, leading
to crashes in ATLFASTIIF and ATLFASTIIF_G4MS simulators.
It is safe to simply set cluster_z
to zero when this occurs.
Fixes: ATLASSIM-4510
Edited by Vincent Pascuzzi