Reparse svgElementsCache instead of deleting it in discardCache

svgElementsCache may be created on render thread and deleted
on main thread, which will make KSharedConfig crash because
it uses per-thread storage.

BUG: 359539
REVIEW: 127112

src/plasma/private/theme_p.cpp

@@ -335,7 +335,7 @@ void ThemePrivate::discardCache(CacheTypes caches)
         discoveries.clear();
         invalidElements.clear();
 
-        svgElementsCache = 0;
+        svgElementsCache->reparseConfiguration();
     }
 }