Fix integer overflow in reverse!

[jishnub]

Move midpoint from Base.Sort to Base and use it to avoid integer overflow. Now

julia> vo = OffsetArray([1:4;], typemax(Int)-4)
4-element OffsetArray(::Vector{Int64}, 9223372036854775804:9223372036854775807) with eltype Int64 with indices 9223372036854775804:9223372036854775807:
 1
 2
 3
 4

julia> reverse!(vo)
4-element OffsetArray(::Vector{Int64}, 9223372036854775804:9223372036854775807) with eltype Int64 with indices 9223372036854775804:9223372036854775807:
 4
 3
 2
 1

Fixes #45870