Fix buffer overflow in rotctl.c

https://github.com/Hamlib/Hamlib/security/code-scanning/3221

tests/rotctl.c

@@ -231,7 +231,7 @@ 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 't':