`getindex` is broken for `Diagonal`

``` julia
julia> D = Diagonal(Int[])
0x0 Diagonal{Int64}

julia> D[-1]
ERROR: integer division error
 in getindex at linalg/diagonal.jl:30

julia> D[-1,-1]
ERROR: BoundsError()
 in getindex at linalg/diagonal.jl:26

julia> D[1,1
       ]
ERROR: BoundsError()
 in getindex at linalg/diagonal.jl:26

julia> D[2]
ERROR: integer division error
 in getindex at linalg/diagonal.jl:30

julia> D[1,-1]
0
```
