SiCombinatorialTrackFinder_xk: Check handles are valid before use

This will reduce the impact of the segfault in ATLASRECTS-8007 by making the affected event fail rather than continuing with a nullptr and then segfaulting (as a silent failure would still mean no tracks for whatever tracking instance was affected) but no way to know this was due to this bug. Athena will not segfault however, so only the affected event will go to debug.

JIRA: ATLASRECTS-8007