GarrisonContain: Add safety checks to prevent crashes when structure or drawable are invalid

[seer-by-sentry]

Fixes CLIENT-S0. The issue was that: GarrisonContain logic called Drawable::clearAndSetModelConditionFlags on a NULL Drawable pointer because the structure object was being destroyed.

• Added null checks for structure and drawable objects before accessing their members or calling methods on them.
• Added checks to ensure structure and drawable are still valid before clearing and setting model condition flags.
• Added checks to ensure structure is still valid before accessing position or calling getMultiLogicalBonePosition.
• Added checks to ensure drawable is still valid before restoring model condition flags.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2486032

Not quite right? Click here to continue debugging with Seer.