dm era: check for a non-NULL metadata object before closing it

era_ctr() may call era_destroy() before era->md is initialized so
era_destory() must only close the metadata object if it is not NULL.

Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Naohiro Aota <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>
Cc: [e-mail] # 3.15+

drivers/md/dm-era-target.c

···
 
 static void era_destroy(struct era *era)
 {
-	metadata_close(era->md);
+	if (era->md)
+		metadata_close(era->md);
 
 	if (era->wq)
 		destroy_workqueue(era->wq);