zink: Fix crash on zink_create_screen error path

zink: Fix crash on zink_create_screen error path

The `zink_internal_create_screen()` function can fail before
`screen->loader_lib` and/or `screen->instance` are initialized.
The `zink_destroy_screen()` doesn't check those cases and crashes.

The error was found by Fedora's CI. The back trace is available at [1].

[1] https://bodhi.fedoraproject.org/updates/FEDORA-2023-c39f82c465

Fixes: 0c2045553fe4 ("zink: use screen destructor for creation fails")
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Signed-off-by: José Expósito <jexposit@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26029>
(cherry picked from commit 8a635e516ebb12411de8055c3405f36be43269b3)