Fixed a problem with the VCS equilibrium solver at constant HP or UP

A sign error when limiting the temperature step caused the solver to fail if
the initial temperature was higher than sqrt(Tlow*Thigh).

Cherry-picked from trunk (r1883).

src/equil/vcs_MultiPhaseEquil.cpp

@@ -293,7 +293,7 @@ int vcs_MultiPhaseEquil::equilibrate_HP(doublereal Htarget,
                 Tnew = sqrt(Tlow*Thigh);
                 dT = Tnew - Tnow;
                 if (dT < -200.) {
-                    dT = 200;
+                    dT = -200;
                 }
                 if (dT > 200.) {
                     dT = 200.;