Minor cosmetic cleanup in class AssociationLine :

umbrello/umlwidgets/associationline.{h,cpp}
- Add prefix "m_" at class members c1dx, c1dy, c2dx, c2dy to align with
  Umbrello code convention.

umbrello/umlwidgets/associationline.cpp
- In function path() return immediately if m_points.count() is 0.
  This saves an indentation level in the nominal execution path.
- In function mouseMoveEvent case (m_activePointIndex != -1),
  - add const int nPoints as shorthand for m_points.size();
  - rewrite if-condition
      (m_activePointIndex == 2)
    as
      (nPoints > 3 && m_activePointIndex == nPoints - 2)
    to avoid the hard coded index 2.