Change Link accces in Matches to at accessor + catch nullptr when matching clusters in MatchRecoToMc, #2003 #2737

Refs #2003 and #2737

• Fixes #2003 by forcing users to check for range before accessing Links (force crash with stack due to uncaught exception)
• Catches (but does not fix) #2737 to provide more meaningful stop message instead of crash with random stack