Fix bug #7263: Instant Preview crash.

Fix bug #7263: Instant Preview crash.

I guess this deserves to be sorted out better, because we are doing tricky things by deleted the object from itself.

Problem:

PreviewImage? has a member PreviewLoader?. PreviewImage::Impl::statusChanged() calls PreviewLoader::remove. PreviewLoader::Impl::remove removes a snippet from the cache. In the cache is a map of the snippet and a shared pointer to PreviewImage?. This means that removing the snippet from the cache, destroys the PreviewImage?. When we get back to PreviewImage::Impl::statusChanged() this will start to crash.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37681 a592a061-630c-0410-9148-cb99ea01b6c8