Fix UndefVarError

[dlfivefifty]

This should fix

julia> f = x -> qr(BandedMatrix(0 => fill(x,10), -1 => ones(9), 1 => ones(9))).R[1,1]
#3 (generic function with 1 method)

julia> f'(0.1)
ERROR: UndefVarError: xs not defined
Stacktrace:
  [1] (::Zygote.var"#442#443")(#unused#::Nothing)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/lib/array.jl:79
  [2] (::Zygote.var"#2351#back#444"{Zygote.var"#442#443"})(Δ::Nothing)
    @ Zygote ~/.julia/packages/ZygoteRules/OjfTt/src/adjoint.jl:59
  [3] Pullback
    @ ./broadcast.jl:894 [inlined]
  [4] Pullback
    @ ./broadcast.jl:891 [inlined]
  [5] Pullback
    @ ./broadcast.jl:887 [inlined]
  [6] (::typeof(∂(materialize!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface2.jl:0
  [7] Pullback
    @ ~/.julia/packages/BandedMatrices/Gbqeq/src/generic/AbstractBandedMatrix.jl:135 [inlined]
  [8] (::typeof(∂(triu!)))(Δ::Zygote.OneElement{Float64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}})
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface2.jl:0
  [9] Pullback
    @ ~/Projects/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/generic.jl:435 [inlined]
 [10] (::typeof(∂(triu!)))(Δ::Zygote.OneElement{Float64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}})
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface2.jl:0
 [11] Pullback
    @ ~/Projects/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/qr.jl:425 [inlined]
 [12] (::typeof(∂(getproperty)))(Δ::Zygote.OneElement{Float64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}})
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface2.jl:0
 [13] Pullback
    @ ~/.julia/packages/ZygoteRules/OjfTt/src/ZygoteRules.jl:11 [inlined]
 [14] Pullback
    @ ./REPL[7]:1 [inlined]
 [15] (::typeof(∂(#3)))(Δ::Float64)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface2.jl:0
 [16] (::Zygote.var"#46#47"{typeof(∂(#3))})(Δ::Float64)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface.jl:41
 [17] gradient(f::Function, args::Float64)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface.jl:76
 [18] (::Zygote.var"#48#49"{var"#3#4"})(x::Float64)
    @ Zygote ~/.julia/packages/Zygote/TaBlo/src/compiler/interface.jl:79
 [19] top-level scope
    @ REPL[8]:1

[DhairyaLGandhi]

Hi, thanks for the contribution!

Could you add a @test_throws with copyto! also?

[CarloLucibello]

coincidentally I just ran in a similar issue, didn't notice this PR, and accidentally pushed the fix on master using the github interface, Sorry for that

[DhairyaLGandhi]

ref f878cf7 which seems to have been committed already.

[mcabbott]

Thanks! Sorry about the bug, this fix is a little nicer than the earlier commit.