ITSMFT: fix clusterizer merge correctly connected pixels

[f3sch]

I just want to check the CI, patch comes from https://its.cern.ch/jira/browse/O2-6424.

[github-actions[bot]]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 18ea114a32e237c339e801dc136b5f928516c6ee at 2026-01-05 11:32:

## sw/BUILD/Herwig-latest/log
make[1]: *** [Makefile:1254: herwigopts.c] Error 127
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:589: all-recursive] Error 1

Full log here.