Fix warnings for zeros(Ptr{T}) and ones(Ptr{T})

This fixes the following warning from Base
```
julia> zeros(Ptr{Void}, 0)
WARNING: (zero{T})(::Type{Ptr{T}}) is deprecated, use (Ptr{T})(0) instead.
 in zeros at array.jl:177
0-element Array{Ptr{Void},1
```