[#482] Fix an exception in ObjectEntity.checkCollision()
[#482] Fix an exception in ObjectEntity.checkCollision()

- See: https://groups.google.com/d/msg/melonjs/w6UawD-I3Dw/SNY1cNy8wcEJ
- Reset the me.game.collisionMap reference to an empty collision map.
- The empty collision map is only created once at startup.

srbala/melonJS,rlugojr/melonJS,melonjs/melonJS,srbala/melonJS,MarkoIvanetic/melonJS,melonjs/melonJS,gopalindians/melonJS,MarkoIvanetic/melonJS,TukekeSoft/melonJS,gopalindians/melonJS,srbala/melonJS,gopalindians/melonJS,TukekeSoft/melonJS,rlugojr/melonJS,rlugojr/melonJS,TukekeSoft/melonJS