fix warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]

[shishirong]

SerialPortYmodem\Ymodem.cpp:1199: warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]
SerialPortYmodem\Ymodem.cpp:1347: warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]
SerialPortYmodem\Ymodem.cpp:1485: warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]
SerialPortYmodem\Ymodem.cpp:1610: warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]
SerialPortYmodem\Ymodem.cpp:1335: warning: Assigning field to itself