NaN returned from Span-Wagner function vapor_pressure

[hflesche]

In the span-wagner CO2 function vapor_pressure a runtime warning is encountered for valid values of temperature and pressure:
RuntimeWarning: invalid value encountered in power
inner = _a1 * _t1.0 + _a2 * _t1.5 + _a3 * _t2.0 + _a4 * _t4.0
but the results of the density and velocity is in accordance with the original publication.

The only input to the function is temperature, and it is normalised against CO2_CRITICAL_TEMPERATURE = 304.1282. The variable _t becomes negative for temperatures above the critical temperature, so the function can generate NaN. This is not caught in the calling function below_vapor_pressure = pressure < vapor_pressure(absolute_temperature)