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 <ejt@redhat.com>
Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.15+

drivers/md/dm-era-target.c

@@ -1391,6 +1391,7 @@ static int era_is_congested(struct dm_target_callbacks *cb, int bdi_bits)
 
 static void era_destroy(struct era *era)
 {
+	if (era->md)
 		metadata_close(era->md);
 
 	if (era->wq)