Fix buffer overflow in rigctld.c

https://github.com/Hamlib/Hamlib/security/code-scanning/3214

tests/rigctld.c

@@ -534,7 +534,7 @@ 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;