Fixed an issue with a undeletable block, the "zero" block

I assume the origin of the if(!vidx), was to catch issues when vidx is undefined or null.
However, it creates a "zero-brick" undeletable bug...

There is a brick that is undeletable, this is because vidx is genuinely equal to 0

For an example, see issue no. 10 in voxel-engine: https://github.com/maxogden/voxel-engine/issues/10