OF-1254 drop ofMucConvLog_msg_id index prior to creating it

This is an attempt to workaround an issue introduced whereby the database schema version is incorrect.  More info is here
https://discourse.igniterealtime.org/t/79916

The mysql change worked for me on mariadb 5.5.56

src/database/upgrade/25/openfire_db2.sql

+DROP INDEX ofMucConvLog_msg_id IF EXISTS;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_hsqldb.sql

+DROP INDEX ofMucConvLog_msg_id IF EXISTS;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_mysql.sql

-ALTER TABLE ofMucConversationLog ADD INDEX ofMucConvLog_msg_id (messageID);
+
+set @exist := (select count(*) from information_schema.statistics where table_name = 'ofMucConversationLog' and index_name = 'ofMucConvLog_msg_id' and table_schema = database());
+set @sqlstmt := if( @exist > 0, 'select ''INFO: Index already exists.''', 'create index ofMucConvLog_msg_id on ofMucConversationLog (messageID)');
+PREPARE stmt FROM @sqlstmt;
+EXECUTE stmt;
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_oracle.sql

+DROP INDEX IF EXISTS ofMucConvLog_msg_id;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_postgresql.sql

+DROP INDEX IF EXISTS ofMucConvLog_msg_id;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_sqlserver.sql

+DROP INDEX IF EXISTS ofMucConvLog_msg_id;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';

src/database/upgrade/25/openfire_sybase.sql

+DROP INDEX IF EXISTS ofMucConvLog_msg_id;
+
 CREATE INDEX ofMucConvLog_msg_id ON ofMucConversationLog (messageID);
 
 UPDATE ofVersion SET version = 25 WHERE name = 'openfire';