fix crash when saving a mesh and a custom property was removed before

extend unittest for this case

git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@1323 fdac6126-5c0c-442c-9429-916003d36597

src/OpenMesh/Core/IO/writer/PLYWriter.cc

@@ -207,7 +207,7 @@ std::vector<_PLYWriter_::CustomProperty> _PLYWriter_::writeCustomTypeHeader(std:
 
 
     // check, if property is persistant
-    if (!prop->persistent())
+    if (!prop || !prop->persistent())
       continue;