fix: createMonsterType memory leak

[jprzimba]

Potential Memory Leak

• The function used lua_error(L) mid-execution. If the Lua/C++ environment is not configured to unwind the C++ stack during a Lua error (which uses longjmp), the destructors of the local objects (std::shared_ptr<MonsterType>, std::set, std::string) would not be called.
• This would cause a leak of the memory allocated for MonsterType and for the strings/sets.

Inconsistent State

• The code attempted to register multiple names (original name, variant, alternative name) in a loop.
• If the first name was successfully registered, but the second failed (already existing), the code called lua_error.

The result would be a "half-registered" monster in the system, but the Lua script would receive an error and stop, leaving the server in a dirty state.

[gabrielew]

using in production for a week

[jprzimba]

using in production for a week

Thank you for your feedback

[QuebradaZN]

working without problems