COBYLA - disabling parameter based stopping criteria results in a negative rhoend

[powel-jeh]

Setting xtol_abs and xtol_rel negative disables the criterions as per: https://nlopt.readthedocs.io/en/latest/NLopt_Reference/#stopping-criteria

However, the code that computes rhoend does not seem to take this into account: https://github.com/stevengj/nlopt/blob/master/src/algs/cobyla/cobyla.c#L221-L225

If both are disabled one ends up with a negative rhoend, which doesn't make sense.