Prevented looking up incorrect object handles

Previously, `dAtoi` would be called on arbitrary strings, which resulted in weird things like `"7.2 3 4.529"` being cast to `7`. Now, `Sim::findObject` checks that object handles (strings starting with a digit) actually only contain digits and slashes.

Refer to [this thread](http://www.garagegames.com/community/forums/viewthread/132371) and [this one](http://www.garagegames.com/community/forums/viewthread/117065).