gdiplus: Check for empty height in bonding box calculation

This resolves the issue in StudioTax where the bounding box dimensions is reported as 0. Because the height is less than 0, the check for height in GdipAddPathRectangle fails, and the X and Y coordinates of the points is never set.

I am fairly ignorant of this code, this seems like a good approach, however I am happy for guidance from others more familiar in how gdiplus works.