Check for payload != NULL in smartPortPayloadContainsMSP()

When calling it from the fport driver, the payload might be NULL in
some cases, causing an invalid dereference.

Found by @jirif on betaflight/betaflight#6060

src/main/telemetry/smartport.c

@@ -233,7 +233,7 @@ void smartPortSendByte(uint8_t c, uint16_t *checksum, serialPort_t *port)
 
 bool smartPortPayloadContainsMSP(const smartPortPayload_t *payload)
 {
- return payload->frameId == FSSP_MSPC_FRAME_SMARTPORT || payload->frameId == FSSP_MSPC_FRAME_FPORT;
+ return payload && (payload->frameId == FSSP_MSPC_FRAME_SMARTPORT || payload->frameId == FSSP_MSPC_FRAME_FPORT);
 }
 
 void smartPortWriteFrameSerial(const smartPortPayload_t *payload, serialPort_t *port, uint16_t checksum)