Fixed returned mails from deleted chars (#7181)

Fixes #7161

Thanks to @Pokye

sql-files/upgrades/upgrade_20220818.sql

@@ -0,0 +1,7 @@
+UPDATE `mail`
+SET `send_id`='0'
+WHERE `send_id` NOT IN (
+	select `char_id`
+	from `char`
+)
+;

src/char/char.cpp

@@ -1729,6 +1729,10 @@ enum e_char_del_response char_delete(struct char_session_data* sd, uint32 char_i
 	if (SQL_ERROR == Sql_Query(sql_handle, "DELETE FROM `%s` WHERE `dest_id`='%d'", schema_config.mail_db, char_id))
 		Sql_ShowDebug(sql_handle);
 
+	/* mark mails as sent from server, if a character gets deleted */
+	if (SQL_ERROR == Sql_Query(sql_handle, "UPDATE `%s` SET `send_id`='0' WHERE `send_id`='%d'", schema_config.mail_db, char_id))
+		Sql_ShowDebug(sql_handle);
+
 #ifdef ENABLE_SC_SAVING
 	/* status changes */
 	if( SQL_ERROR == Sql_Query(sql_handle, "DELETE FROM `%s` WHERE `account_id` = '%d' AND `char_id`='%d'", schema_config.scdata_db, account_id, char_id) )