fix crash on datum point preselect

It is unclear why the coordinates are NULL. This fix prevents the crash but does not fix the reason why this unnormality occurs.

src/Mod/Part/Gui/SoBrepPointSet.cpp

@@ -112,6 +112,8 @@ void SoBrepPointSet::renderShape(const SoGLCoordinateElement * const coords,
                                  int numindices)
 {
     const SbVec3f * coords3d = coords->getArrayPtr3();
+    if(coords3d == nullptr)
+        return;
 
     int previ;
     const int32_t *end = cindices + numindices;