CombinedMuonTrackBuilder: Check pointer equality to avoid occasional double-delete

Walter Lamplrequested to merge
wlampl/athena:fix5839 into master

One of the many possible code-branches in CombinedMuonTrackBuilder::standaloneFit may lead to the two pointer variables "track" and "extrapolated" pointing to the same object. This change fixes the double-delete.

Fixes ATLASRECTS-5839.

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