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>

src/gallium/drivers/d3d12/d3d12_screen.cpp

@@ -743,7 +743,8 @@ d3d12_destroy_screen(struct d3d12_screen *screen)
    slab_destroy_parent(&screen->transfer_pool);
    mtx_destroy(&screen->submit_mutex);
    mtx_destroy(&screen->descriptor_pool_mutex);
-   util_dl_close(screen->d3d12_mod);
+   if (screen->d3d12_mod)
+      util_dl_close(screen->d3d12_mod);
    glsl_type_singleton_decref();
    FREE(screen);
 }