Typo when edge in polygon is parallel to the ray in point_in_polygon of CSGCGALDomain2D.cpp

There is a typo in point_in_polygon (CSGCGALDomain2D.cpp, https://bitbucket.org/fenics-project/mshr/src/master/src/CSGCGALDomain2D.cpp ) in which the wrong pointer is referenced. Depending on the list of coordinates, in the case when an edge in the polygon is parallel to the ray, this results in a "Segmentation Fault". Here follows a patch that would fix this typo of the referenced variables.