[Crash] Check mob pointer before trying to remove it

[Akkadius]

Description

This attempts to check a pointer before trying to do book keeping. We are observing a crashing during zone shutdown during RemoveAllMobs that is unclear the exact source of the trigger

Attempts to fix crash fingerprint 10444 which has over 100 occurrences at this point https://spire.akkadius.com/dev/release/22.48.0?id=20718

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

Had 3 zones shutdown that trigger RemoveAllMobs

• Currently running and baking on PEQ. Seems to have helped the crashes.
• This should be tested and baked on a server with players such as PEQ to confirm it doesn't create other issues

Clients tested: ROF2

Checklist

• I have performed a self-review of my code. Ensuring variables, functions and methods are named in a human-readable way, comments are added only where naming of variables, functions and methods can't give enough context.
• I have tested my changes
• I have made corresponding changes to the documentation
• I own the changes of my code and take responsibility for the potential issues that occur