Clean up a bit iterator management in oox/vmldrawing

Change-Id: Icd4f4980332edc3fd9027cc4cace2b4a0d0c1315
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/123234
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

oox/source/vml/vmldrawing.cxx

@@ -196,10 +196,10 @@ void Drawing::convertAndInsert() const
             {
                 RadioItr->first->setPropertyValue("GroupName", aGroup );
                 // If conflict, first created GroupBox wins
-                RadioButtonMap.erase( RadioItr++ );
+                RadioItr = RadioButtonMap.erase(RadioItr);
             }
             else
-                RadioItr++;
+                ++RadioItr;
         }
     }