MT#63239 Add IF NOT EXISTS option when creating INDEX for 15841.down

By default, Prosody automatically handles table management.
As a result, executing the down script will fail because the index
has already been created by Prosody.

Adding IF NOT EXISTS prevents errors when the index already exists.

Change-Id: Ie9289ec5381fd1082047e5c9a762e7a31cf1c55c

db_scripts/diff/15841.down

@@ -2,5 +2,5 @@ USE prosody;
 SET autocommit=0;
 
 DROP INDEX prosody_unique_index on prosody;
-CREATE INDEX prosody_index ON prosody(`host`(20), `user`(20), `store`(20), `key`(20)) USING BTREE;
+CREATE INDEX IF NOT EXISTS prosody_index ON prosody(`host`(20), `user`(20), `store`(20), `key`(20)) USING BTREE;
 COMMIT;