Fix lmul!/rmul! for 0-sized matrices (#1290)

Fixes scaling `Bidiaognal` matrices by `Diagonal` ones when they're both
empty.

Author: jishnub

src/bidiag.jl

@@ -538,8 +538,9 @@ function lmul!(D::Diagonal, B::Bidiagonal)
     matmul_size_check(size(D), size(B))
     (; dv, ev) = B
     isL = B.uplo == 'L'
-    dv[1] = D.diag[1] * dv[1]
-    for i in axes(ev,1)
+    iszero(size(D,1)) && return B
+    @inbounds dv[1] = D.diag[1] * dv[1]
+    @inbounds for i in axes(ev,1)
         ev[i] = D.diag[i + isL] * ev[i]
         dv[i+1] = D.diag[i+1] * dv[i+1]
     end
@@ -575,8 +576,9 @@ function rmul!(B::Bidiagonal, D::Diagonal)
     matmul_size_check(size(B), size(D))
     (; dv, ev) = B
     isU = B.uplo == 'U'
-    dv[1] *= D.diag[1]
-    for i in axes(ev,1)
+    iszero(size(D,1)) && return B
+    @inbounds dv[1] *= D.diag[1]
+    @inbounds for i in axes(ev,1)
         ev[i] *= D.diag[i + isU]
         dv[i+1] *= D.diag[i+1]
     end

test/bidiag.jl

@@ -1192,4 +1192,13 @@ end
     @test_throws msg ldiv!(C, B, zeros(2,1))
 end
 
+@testset "l/rmul with 0-sized matrices" begin
+    n = 0
+    B = Bidiagonal(ones(n), ones(max(n-1,0)), :U)
+    B2 = copy(B)
+    D = Diagonal(ones(n))
+    @test lmul!(D, B) == B2
+    @test rmul!(B, D) == B2
+end
+
 end # module TestBidiagonal