Resolve undefined behavior with VLAs length

RecoEcal/EgammaClusterAlgos/src/Multi5x5BremRecoveryClusterAlgo.cc:47:34:
runtime error: variable length array bound evaluates to non-positive
value 0
RecoEcal/EgammaClusterAlgos/src/Multi5x5BremRecoveryClusterAlgo.cc:50:30:
runtime error: variable length array bound evaluates to non-positive
value 0

Multi5x5BremRecoveryClusterAlgo::makeIslandSuperClusters(..) is called
with empty clusters_v. There is nothing to be done if clusters_v is
empty, thus bail out early.

This happens 36 times while running short matrix.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #5424 from davidlt/ubsan-reco-vla-1

Resolve undefined behavior with VLAs length