AP_HAL: fixed a logic bug in the srxl state machine

when decoding invalid input the buflen could grow to beyond the buffer
size, overwriting the next block of memory. On Disco this ended up
overwriting the Perf::_instance variable, which led to a segmentation
fault

libraries/AP_HAL/utility/srxl.cpp

@@ -284,8 +284,8 @@ int srxl_decode(uint64_t timestamp_us, uint8_t byte, uint8_t *num_values, uint16
             frame_len_full = 0U;
             frame_header = SRXL_HEADER_NOT_IMPL;
             decode_state = STATE_IDLE;
-            ret = 2; /* protocol version not implemented --> no channel data --> unknown packet  */
-            break;
+            buflen = 0;
+            return 2; /* protocol version not implemented --> no channel data --> unknown packet  */
         }
     }
 
@@ -300,6 +300,14 @@ int srxl_decode(uint64_t timestamp_us, uint8_t byte, uint8_t *num_values, uint16
         break;
 
     case STATE_COLLECT: /* receive all bytes. After reception decode frame and provide rc channel information to FMU   */
+        if (buflen >= frame_len_full) {
+            // a logic bug in the state machine, this shouldn't happen
+            decode_state = STATE_IDLE;
+            buflen = 0;
+            frame_len_full = 0;
+            frame_header = SRXL_HEADER_NOT_IMPL;
+            return 2;
+        }
         buffer[buflen] = byte;
         buflen++;
         /* CRC not over last 2 frame bytes as these bytes inhabitate the crc */