Fix indexing for offset views of BandedMatrixes

[jishnub]

On master

julia> B = BandedMatrix(0=>1:4)
4×4 BandedMatrix{Int64} with bandwidths (0, 0):
 1  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4

julia> view(B, Base.IdentityUnitRange(2:4), 1:4)
3×4 view(::BandedMatrix{Int64, Matrix{Int64}, Base.OneTo{Int64}}, Base.IdentityUnitRange(2:4), 1:4) with eltype Int64 with indices 2:4×Base.OneTo(4):
 ⋅  ⋅  0  ⋅
 ⋅  ⋅  ⋅  0
 ⋅  ⋅  ⋅  ⋅

This PR

julia> view(B, Base.IdentityUnitRange(2:4), 1:4)
3×4 view(::BandedMatrix{Int64, Matrix{Int64}, Base.OneTo{Int64}}, Base.IdentityUnitRange(2:4), 1:4) with eltype Int64 with indices 2:4×Base.OneTo(4):
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.57%. Comparing base (13b11fc) to head (3d4683b).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #432      +/-   ##
==========================================
- Coverage   89.59%   89.57%   -0.03%     
==========================================
  Files          25       25              
  Lines        3557     3559       +2     
==========================================
+ Hits         3187     3188       +1     
- Misses        370      371       +1     

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