Fix linear indexing of one-dimensional CartesianIndices

[timholy]

This fixes some surprising outcomes:

julia> ci = CartesianIndices((2:4,))
3-element CartesianIndices{1,Tuple{UnitRange{Int64}}}:
 CartesianIndex(2,)
 CartesianIndex(3,)
 CartesianIndex(4,)

julia> show(ci)
CartesianIndex{1}[CartesianIndex(3,), CartesianIndex(4,), #undef]
julia> collect(ci)
3-element Array{CartesianIndex{1},1}:
 CartesianIndex(2,)
 CartesianIndex(3,)
 CartesianIndex(4,)

julia> collect(LinearIndices(ci))
3-element Array{Int64,1}:
 1
 2
 3

julia> for i in LinearIndices(ci)
           @show i
       end
i = 2
i = 3
i = 4

CC @mbauman

[mbauman]

Mac failure is the old FileWatching issue, #26725.