Fix georeference code

I was trying to georeference some images using the APM log and it wasn't working.  It seams that the problem was that the positions from where you are getting the data are wrong, it also give me an error when trying to georef since it tried to get a double from the string "GPS:"

This fix pull request fix the problem. My setup is:
Mission Planner v1.2.48
ArduPlane v2.72