TauRecTools: Check CellLinks before use in TauPi0ClusterCreator
A segmentation fault was identified when running the TauAODReReco chain [1] in AOD to DAOD_PHYS stage [2].
This patch is to fix the rare segfault by checking the links before use.
[1]atlas/athena!52225 (merged) [2]https://its.cern.ch/jira/browse/ATLASG-1637.