1-​Piperazinecarboxylic acid, (5S)​-​6-​(5-​chloro-​2-​pyridinyl)​-​6,​7-​dihydro-​7-​oxo-​5H-​pyrrolo[3,​4-​b]​pyrazin-​5-​yl ester, hydrochloride (1:1)