fix memory corruption in deleteat!

[vtjnash]

n.b. n == a->nrows + dec, so we overwrote dec bytes of unrelated memory here

Fixes JuliaData/DataFrames.jl#2819

Hard to test for memory corruption mechanically, but we can trigger the failure via:

julia> a = Vector{Union{Bool,Missing}}(missing, 10^4);                                                                                                                                                   
julia> deleteat!(a, 2:(length(a) - 5));                                                                                                                                                                  
julia> a                                                                                                                                                                                                 
corrupted double-linked list                                                                                                                                                                             

[bkamins]

I confirm that with this patch all works correctly. I understand that this will get backported to 1.6 (if there is any new release) and 1.7, as in DataFrames.jl I assumed that the work-around for this bug is only needed for Julia 1.6.2 or earlier.

Thank you!