d3d12: Don't crash when libd3d12.so can't be found

d3d12: Don't crash when libd3d12.so can't be found

`d3d12_destroy_screen` is called by `d3d12_create_dxcore_screen` after
`d3d12_init_screen_base` fails and attempts to call `util_dl_close` on
a NULL pointer, leading to an abort.

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 b3d1ae19f2f4d93cf0a5f45a598149ac4e8e05aa)