Fix a crash when npc tries to heal horse-mounted player

[Qrox]

Summary

SUMMARY: Bugfixes "Fix a crash when npc tries to heal horse-mounted player."

Purpose of change

Fixes #40001.

game::shared_from assumed that the player cannot be at the same location as a monster, which no longer holds now that mounting creatures is possible. If the player is mounted on a creature g->shared_from( g->u ) returns the mounted creature, so patient in npc::execute_action becomes nullptr after dynamic casting it to player.

Describe the solution

In npc_execute_action, check patient is not null before dereferencing it.

In game::shared_from, check critter is a monster/npc before searching for its shared pointer, and only return the shared pointer if it refers to the same object as critter.

Testing

Loaded the save from #40001 and it no longer crashed. NPCs now also heal the player when they are mounted.

Additional context

Should probably be backported to 0.E.

[jbytheway]

I don't think this is a good solution; it makes this function O(n) in the number of monsters, which could have a bad performance impact.

I think it would also work to instead replace the initial if block with:

    if( const shared_ptr_fast<monster> mon_ptr = critter_tracker->find( critter.pos() ) ) {
        if( shared_ptr<T> result = std::dynamic_pointer_cast<T>( mon_ptr ) ) {
            return result;
        }
    }

Then for this case (player riding monster) the cast would fail and the code would fall through to the next check and succeed in returning the player.