Sketch: Bug fix: On delete crash with unsuccesful solving

==========================================================

Fixes the one reported by JMG here (Thanks!!):
http://forum.freecadweb.org/viewtopic.php?f=3&t=11508#p92693

Why?
- The newly introduced redrawing policy requires that the solver geometry matches in number the sketchobject geometry.
- Most (all) problems with updateColor or getGeometry returning a null pointer are related to an out of sync between UI geometry and SketchObject geometry.

General solution:
- In other bugs, a missing "solve()" is the problem, once the solver and sketchobject geometries have the same number, an SketchObject::OnChanged triggers
a VPSketch::draw (via VPSketch::updateData) which updates the UI geometry to match SketchObject geometry and then the problem does not arise.

Particular solution:
- In this bug, the problem is not a missing solve, but the fact that the solving was not succesful and did not synchronize the geometries, however triggering a
draw() on unsuccessful solving, syncronizes the UI geometry with the SketchObject geometry and the crash is gone.

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -4785,10 +4785,18 @@ bool ViewProviderSketch::onDelete(const std::vector<std::string> &subList)
             }
         }
         
-        getSketchObject()->solve();
+        int ret=getSketchObject()->solve();
+        
+        if(ret!=0){
+            // if the sketched could not be solved, we first redraw to update the UI geometry as
+            // onChanged did not update it.
+            UpdateSolverInformation();
+            draw();
+        }
         
         this->drawConstraintIcons();
         this->updateColor();
+
         // if in edit not delete the object
         return false;
     }