Warn about loading dead bodies, but handle safely.

[fluffyfreak]

Save games could fail to load due to the body index being written as (Uint32)-1.
Handle getting an out-of-range / SDL_MAX_UINT32 value when trying to find a body by returning nullptr instead.

This allows savegames to load despite this issue.

The issue itself is caused by the AICmdFlyAround having a pointer to a Body *m_obstructor that it does not own and is not refcounted. This can be destroyed before/whilst saving, which fails to find the body ('cos it's gone), the GetIndexForBody returns (Uint32)-1, and upon loading it crashes.

I would like to have fixed the actual cause by this COVID19 vaccination has hit me like a bus and I feel like I've got flu.
So posting this as-is for possible merging, comments, and suggestions. Either other can fix it, or it can wait until my brain works again.

[Web-eWorks]

I would prefer returning INT32_MAX (and in general avoiding all SDL int-types in the codebase) but this is good to merge now and we can convert it to standard types at a later date.