util/foz: stop crashing on destroy if prepare hasn't been called

Fixes: eca6bb9540 ("util/fossilize_db: add basic fossilize db util to read/write shader caches")

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11931>

src/util/fossilize_db.c

@@ -334,7 +334,8 @@ foz_prepare(struct foz_db *foz_db, char *cache_path)
 void
 foz_destroy(struct foz_db *foz_db)
 {
-   fclose(foz_db->db_idx);
+   if (foz_db->db_idx)
+      fclose(foz_db->db_idx);
    for (unsigned i = 0; i < FOZ_MAX_DBS; i++) {
       if (foz_db->file[i])
          fclose(foz_db->file[i]);