[BUG] triu returns wrong matrix when M<N and k<0

[GiggleLiu]

When I intended to implement rq decomposition, I finded this bug in triu! function by accident.
When the input matrix size (M, N) with M>N, for k<0, the following behavior is not correct

e.g.

julia> mat=[1 2;
            3 4;
            5 6;
            7 8]
4×2 Array{Int64,2}:
 1  2
 3  4
 5  6
 7  8

julia> triu!(mat, -1)
4×2 Array{Int64,2}:
 1  2
 3  4
 0  6
 0  8

which is wrong, it should be

[1 2;
 3 4;
 0 6;
 0 0]

Here, min(n, n + k) should be replaced by min(n, m + k)

julia/stdlib/LinearAlgebra/src/dense.jl

Line 174 in 592a474

for j in 1:min(n, n + k)

A PR is ready to fix this.

[staticfloat]

Closed by #29712