dzn: Don't crash when libd3d12.so can't be found

dzn: Don't crash when libd3d12.so can't be found

`dzn_instance_create` will call `dzn_instance_destroy` when the d3d12
library fails to load. Just like the issue in `d3d12_screen`, this will
lead to a crash because `d3d12_mod` is NULL.

To fix this, only close the library after if it was actually opened.

Cc: mesa-stable
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20145>
(cherry picked from commit b5133894005720db24a8e0cc17e047a291953ff4)