Fix for obj_1.depth = 10; crashing when obj_1 isn't in the room.

Say you have the following in any object except obj_1:
```
obj_1.depth = 100;
```

The default object returned does not have a valid iterator set in depth, so depthv::function() will segfault. I fixed this by adding a default constructor that nulls out "myiter", and then just checking that.

I also tested various other "multifunction_variants", such as speedv, hspeedv, etc. None of them seemed to crash.

This crash occurs because the default variable never has init() called for its depth parameter. I don't think tracking depth for the default variant makes a whole lot of sense, so I just had the function return early if no object was bound.