Core/Guild: Fix Guild validation crash on start up

Partially revert 58ec4e531914121144b6f852e3abf27cbb2f6fc4 and increment GuildStore iterator before validating Guilds. This is required because Guild::Validate() removes the Guild from GuildStore if invalid.
Closes #8363
Merge pull request #10722 from jackpoz/guild_startup_crashfix

Core/Guild: Fix Guild validation crash on start up