App: Bugfix for NULL-Pointer dereference of Property->getName()

Some parts of FreeCAD use prop->getName() without testing
for NULL first, e.g.:

src/Mod/PartDesign/Gui/ViewProviderAddSub.cpp:258
 ViewProviderAddSub::updateData()
src/Mod/PartDesign/Gui/ViewProviderDatumCS.cpp:193
 ViewProviderDatumCoordinateSystem::updateData()

See also:
https://forum.freecadweb.org/viewtopic.php?f=3&t=58603

src/App/Property.cpp

@@ -62,6 +62,8 @@ Property::~Property()
 
 const char* Property::getName(void) const
 {
+    if (myName == NULL)
+        return "?";
     return myName;
 }