Fix buffer overflow in rigctl.c

https://github.com/Hamlib/Hamlib/security/code-scanning/3212

tests/rigctl.c

@@ -475,7 +475,7 @@ int main(int argc, char *argv[])
                 return 1;
             }
 
-            strncat(conf_parms, optarg, MAXCONFLEN - strlen(conf_parms));
+            strncat(conf_parms, optarg, MAXCONFLEN - strlen(conf_parms) - 1);
             break;
 
         case 'o':