Ignore empty resource items in plClientResMgr list on destruction.

Sources/Plasma/PubUtilLib/plClientResMgr/plClientResMgr.cpp

@@ -54,6 +54,7 @@ plClientResMgr::~plClientResMgr()
         std::map<std::string, plMipmap*>::iterator it;
 
         for (it = this->ClientResources->begin(); it != this->ClientResources->end(); ++it) {
+            if (it->second)
                 it->second->UnRef();
         }