Fix ASSERT in zil_create() during ztest

This patch corrects an ASSERT in zil_create() that will only be
true if the call to zio_alloc_zil() does not fail.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8010

module/zfs/zil.c

@@ -720,7 +720,8 @@ zil_create(zilog_t *zilog)
 		txg_wait_synced(zilog->zl_dmu_pool, txg);
 	}
 
-	ASSERT(bcmp(&blk, &zh->zh_log, sizeof (blk)) == 0);
+	ASSERT(error != 0 || bcmp(&blk, &zh->zh_log, sizeof (blk)) == 0);
+	IMPLY(error == 0, lwb != NULL);
 
 	return (lwb);
 }