Remove undefined behavior from PixelTrackCleanerBySharedHits.cc

According to UBSan

    RecoPixelVertexing/PixelTrackFitting/src/
    PixelTrackCleanerBySharedHits.cc:38:20

is triggered 151 times on full matrix.

Input vector for the function is empty, which attempts to create
0-length VLA. There is nothig to be done if size is 0, bail out early.

Additionaly make sure that all indexes are unsigned.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>