Part of the problem with calling something “a must win” is what happens when you don’t actually win the game.David Pastrnak and the Boston Bruins found that out the hard way in Tuesday night’s final home game prior to the NHL’s three-day holiday break where the bottom completely fell out in the third period of a 6-2 loss to the Montreal Canadiens at TD Garden.A broken stick for Nikita Zadorov at a key juncture in the third period led to Montreal’s game-winning goal for Zac Bolduc, and then a pair of ensuing 5-on-3 power plays for the Canadiens pushed things out of reach as the Habs scored four unanswered goals in a third period that’s usually Boston’s strong suit.