`getindex(::Sparse, ...)` returns incorrect results for unsorted `Sparse` objects

[topolarity]

julia> using SparseArrays, SparseArrays.CHOLMOD

julia> A = sparse([2, 1, 2], [1, 2, 2], Float64[1, 2, 3])

2×2 SparseMatrixCSC{Float64, Int64} with 3 stored entries:
  ⋅   2.0
 1.0  3.0

julia> S = CHOLMOD.Sparse(A); P = S' * S; # P ends up unsorted

julia> Matrix(sparse(P)) # good
2×2 Matrix{Float64}:
 1.0   3.0
 3.0  13.0

julia> P[1,2]
0.0

this affects some conversions as well:

julia> Array(P) # bad
2×2 Matrix{Float64}:
 1.0  0.0
 3.0  0.0

noticed while investigating the bug in #756

[topolarity]

Goes back to at least Julia 0.6...