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
DB/Misc: Fix few startup errors
Core/Achievements: Converted save queries to prepared statements
Core/Vehicles: Add vehicle_seat_flag_unk18 (0x20000) to seat.IsUsableByOverride

Exact definition still needs to be researched
Scripts/Commands: Check if creature entry is valid before attempting to spawn (fixes unneccessary error in logs)
Update DBCStructure.h