make sure packet size is multiple of 16 bytes

SSUSession.cpp

@@ -1094,6 +1094,8 @@ namespace transport
 	{
 		uint8_t buf[SSU_MTU_V4 + 18];
 		size_t msgSize = len + sizeof (SSUHeader); 
+		size_t paddingSize = msgSize >> 4; // %16
+		if (paddingSize > 0) msgSize += (16 - paddingSize);
 		if (msgSize > SSU_MTU_V4)
 		{
 			LogPrint (eLogWarning, "SSU payload size ", msgSize, " exceeds MTU");