fix segfault in BuiltinElement::dispose

If a BuiltinElement is never used (i.e. we never try to load anything
from it), its init method will never be called, and thus its backing
library will never be loaded.  In that case, we should not try to call
library->disposeAll() in the destructor.
Merge pull request #336 from dicej/builtinelement-dispose

fix segfault in BuiltinElement::dispose