Handle out-of-bounds blockcolstop/blockrowstop

[jishnub]

The following works after this:

julia> using BlockBandedMatrices, LinearAlgebra

julia> B = BlockSkylineMatrix{Bool}(I, 1:1, 1:5, ([0,0,0,0,0],[0,1,1,1,1]))
1×5-blocked 1×15 BlockSkylineMatrix{Bool, Vector{Bool}, BlockBandedMatrices.BlockSkylineSizes{Tuple{BlockArrays.BlockedUnitRange{ArrayLayouts.RangeCumsum{Int64, UnitRange{Int64}}}, BlockArrays.BlockedUnitRange{ArrayLayouts.RangeCumsum{Int64, UnitRange{Int64}}}}, Vector{Int64}, Vector{Int64}, BandedMatrices.BandedMatrix{Int64, Matrix{Int64}, Base.OneTo{Int64}}, Vector{Int64}}}:
 1  │  0  0  │  ⋅  ⋅  ⋅  │  ⋅  ⋅  ⋅  ⋅  │  ⋅  ⋅  ⋅  ⋅  ⋅

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.22%. Comparing base (7df446b) to head (36d3b81).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #192 +/- ## ========================================== - Coverage 88.37% 88.22% -0.15% ========================================== Files 11 11 Lines 1204 1206 +2 ========================================== Hits 1064 1064 - Misses 140 142 +2 ```

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