+(::StridedMatrix, ::BlockDiagonal) fails for non-square blocks

[mjp98]

Example:

julia> A = rand(1,2)
1×2 Matrix{Float64}:
 0.856573  0.994251

julia> A + BlockDiagonal([A])
ERROR: DimensionMismatch: array could not be broadcast to match destination
Stacktrace:
  [1] check_broadcast_shape
    @ ./broadcast.jl:540 [inlined]
  [2] check_broadcast_shape
    @ ./broadcast.jl:541 [inlined]
  [3] check_broadcast_axes
    @ ./broadcast.jl:543 [inlined]
  [4] check_broadcast_axes
    @ ./broadcast.jl:547 [inlined]
  [5] instantiate
    @ ./broadcast.jl:284 [inlined]
  [6] materialize!
    @ ./broadcast.jl:871 [inlined]
  [7] materialize!
    @ ./broadcast.jl:868 [inlined]
  [8] +(B::BlockDiagonal{Float64, Matrix{Float64}}, M::Matrix{Float64})
    @ BlockDiagonals ~/.julia/dev/BlockDiagonals.jl/src/base_maths.jl:39
  [9] +(M::Matrix{Float64}, B::BlockDiagonal{Float64, Matrix{Float64}})
    @ BlockDiagonals ~/.julia/dev/BlockDiagonals.jl/src/base_maths.jl:26
 [10] top-level scope
    @ REPL[23]:1

The code here implicitly assumes square blocks:

    row = 1
    for (j, block) in enumerate(blocks(B))
        nrows = size(block, 1)
        rows = row:(row + nrows-1)
        A[rows, rows] .+= block
        row += nrows
    end