Fix segfault with FS+FR dev (nov22 v18.8.x) at end of run_unpack macros exec
Fixes crash identified by @a.toia and confirmed through introduction of Nightly test of the DEV FS+FR with mCBM data (cf https://cdash.gsi.de/buildSummary.php?buildid=414879)
May still need further tuning (as well as in CbmRun class) when we switch to latest FR minor branch
Redmine Refs #2573