MuonTrackSteering: Check validity of return value from m_candidateTool->createSegInfo

Walter Lamplrequested to merge
wlampl/athena:fixMuoCandidateSegfault into 24.0

Fixes the segfaults observed in the express processing of run 497457. Closes ATLASRECTS-8213

cc @dta @jdevivi

Edited by Patrick Scholer

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