Update constraint.lua

A fix for 
```

[ERROR] lua/includes/modules/constraint.lua:166: Tried to use invalid object (type IPhysicsObject) (Object was NULL or not of the right type)
  1. EnableCollisions - [C]:-1
   2. SetPhysicsCollisions - lua/includes/modules/constraint.lua:166
    3. RemoveAll - lua/includes/modules/constraint.lua:230
     4. DoRemoveEntity - gamemodes/sandbox/entities/weapons/gmod_tool/stools/remover.lua:21
      5. LeftClick - gamemodes/sandbox/entities/weapons/gmod_tool/stools/remover.lua:52
       6. unknown - gamemodes/sandbox/entities/weapons/gmod_tool/shared.lua:254
```
Merge pull request #702 from robotboy655/patch-3

Update constraint.lua