Fixed dither issue where when star is not found, large drift is wrongly calculated.

This fixes an issue caused by the fact pmath->getStarScreenPosition() is called in internalguider.cpp several times without checking to see if pmath->isStarLost(). The guidestar position, when the star is lost, is returned as -1,-1, but, of course, one should not use that value as a real star position.

There are other instances of using this without check in this file that should be fixed, but getting this one out ASAP.