icon-theme: Fix deadlock in GktIconTheme dispose

icon-theme: Fix deadlock in GktIconTheme dispose

When freeing the display GtkIconTheme and that was the last owner we
ran into a deadlock, because we unref the "next-to-last" ref inside a
gtk_icon_theme_ref_aquire/release() pair, which makes the final unref
to happen in the release(), while the ref lock still was held.
The unref triggers dispose which tries to NULL out the ref, but that then
deadlocks on the mutex being held by the caller already.

We fix this by moving the release unref outside the lock. This is safe
because refcounts are atomic, and we *do* own the ref.