storage: file: Tolerate NULL src when uninitializing the backend

Allow de-init of null storage sources.

src/storage/storage_driver.c

@@ -2540,7 +2540,7 @@ int storageRegister(void)
 static bool
 virStorageFileIsInitialized(virStorageSourcePtr src)
 {
-    return !!src->drv;
+    return src && src->drv;
 }