CoolProp returns same value for p_critical and p_triple for R503

[mapipolo]

I contacted Eric Lemmon at NIST first about this, as I expected that this was more likely something in REFPROP (and possibly related to #874), but he doubts it's a REFPROP bug:

AbstractState s = AbstractState.factory(“REFPROP”, “R503.mix”);
s.p_critical(); // --> 4281310 (correct) 
s.p_triple(); // --> 4281310

Using 1/20 nightly build, REFPROP 9.12.