parse3Rat2 returns NaN rather than error or 0.0

[ehalpern]

I've encountered several photos that have an exif LatLong entry (encoded as rationals) where one or both of the coordinates have numerator == denominator == 0. In this case exif.LatLong() returns NaN for the offending point(s).

If would be helpful if this case were handled more directly. Perhaps you could return an error in any case where the denominator is 0. Additionally, you could interpret any rational with a 0 numerator as 0.0.

[xor-gate]

Feel free to suggest a patch as Pull Request, I prefer also with a unit test picture.