dot(x,A,x) fails for 1x1 SymTridiagonal

[stevengj]

Bug found by @smataigne:

julia> dot([1], SymTridiagonal([1],Int[]), [1])
ERROR: BoundsError: attempt to access 1-element Vector{Int64} at index [2]
Stacktrace:
 [1] getindex
   @ ./array.jl:924 [inlined]
 [2] dot(x::Vector{Int64}, S::SymTridiagonal{Int64, Vector{Int64}}, y::Vector{Int64})
   @ LinearAlgebra /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/LinearAlgebra/src/tridiag.jl:265

This line assumes the matrix is at least 2x2: https://github.com/JuliaLang/julia/blob/18fa3835a78bdbe5982daf32b8aa84ab7fd2362e/stdlib/LinearAlgebra/src/tridiag.jl#L266-L267

Recommendation: change this check to:

 if nx ≤ 1
    nx == 0 && return dot(zero(eltype(x)), zero(eltype(S)), zero(eltype(y)))
    return dot(x[1], S.dv[1], y[1])
end

(Rationale: the common case is nx > 1, so better to have a single check nx ≤ 1 in that circumstance than separate nx == 0 and nx == 1 checks.)

Just needs someone to create a PR with this text and add a patch.

cc @dkarrasch who added this code in JuliaLang/julia#32739.