AP_RCProtocol: fixed crc memory range error in CRSF

this fixes a crash bug that caused a watchdog for Henry on a
F765-Wing. The bug happens with corrupt serial data causing an
underflow in the length argument to the crc call

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.cpp

@@ -276,6 +276,11 @@ bool AP_RCProtocol_CRSF::check_frame(uint32_t timestamp_us)
         return false;
     }
 
+    if (_frame.length + CRSF_HEADER_LEN < _frame_ofs) {
+        // invalid short frame
+        return false;
+    }
+
     // decode whatever we got and expect
     if (_frame_ofs >= _frame.length + CRSF_HEADER_LEN) {
         const uint8_t crc = crc8_dvb_s2_update(0, &_frame_bytes[CRSF_HEADER_LEN], _frame.length - 1);

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.h

@@ -31,6 +31,7 @@
 #define CRSF_FRAMELEN_MAX   64U      // maximum possible framelength
 #define CRSF_HEADER_LEN     2U       // header length
 #define CRSF_FRAME_PAYLOAD_MAX (CRSF_FRAMELEN_MAX - CRSF_HEADER_LEN)     // maximum size of the frame length field in a packet
+#define CRSF_FRAME_LENGTH_MIN 2 // min value for _frame.length
 #define CRSF_BAUDRATE      416666U
 #define ELRS_BAUDRATE      420000U
 #define CRSF_TX_TIMEOUT    500000U   // the period after which the transmitter is considered disconnected (matches copters failsafe)