fix bug in encrypt
fix bug in encrypt

Note that the pad is present when M is evenly divisible by 16. EXAMPLE A 9-byte message has a pad of 7 bytes, each with the value 0x07.

ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core,ONLYOFFICE/core