Broken UI because of exceptions during DPI change event processing

[HeikoKlare]

I ran into a broken UI after processing a DPI change event because a disposed image was tried to be set, see: eclipse-platform/eclipse.platform.swt#2432

This was caused by a disposed image being set. It shall be ensure that the image being set is not disposed to avoid an exception. It may even directly be ensures that no DPI change handler throwing an exception may lead to a broken UI at all.