svtool/GraphicManager: properly decrease mnUsedSize on unregister ops

mnUsedSize was never decreased causing the cache to pretending to be full,
which caused lots of needless work (image loading, id creation (which are
basically a crc of all bytes of an image, ...))

Change-Id: I1e0a6a07631435346c3509822fae43a067d33303
Reviewed-on: https://gerrit.libreoffice.org/14389Reviewed-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Zolnai Tamás <zolnaitamas2000@gmail.com>
Tested-by: Zolnai Tamás <zolnaitamas2000@gmail.com>

svtools/source/graphic/grfmgr2.cxx

@@ -167,12 +167,13 @@ void GraphicManager::ImplUnregisterObj( const GraphicObject& rObj )
     {
         if ( *it == &rObj ) {
             maObjList.erase( it );
+
+            if( !rObj.IsSwappedOut() )
+                mnUsedSize -= rObj.GetSizeBytes();
             return;
         }
     }
     assert(false); // surely it should have been registered?
-    if( !rObj.IsSwappedOut() )
-        mnUsedSize -= rObj.GetSizeBytes();
 }
 
 void GraphicManager::ImplGraphicObjectWasSwappedOut( const GraphicObject& rObj )