fix herk with complex α (#1297)

Fixes a bug that I introduced in #1249. Sorry about that.

---------

Co-authored-by: Jishnu Bhattacharya <jishnub.github@gmail.com>

Author: araujoms

src/matmul.jl

@@ -820,7 +820,7 @@ Base.@constprop :aggressive function herk_wrapper!(C::StridedMatrix{TC}, tA::Abs
     end
 
     # BLAS.herk! only updates hermitian C, alpha and beta need to be real
-    if iszero(β) || ishermitian(C)
+    if isreal(α) && isreal(β) && (iszero(β) || ishermitian(C))
         alpha, beta = promote(α, β, zero(T))
         if (alpha isa T && beta isa T &&
                 stride(A, 1) == stride(C, 1) == 1 &&

test/matmul.jl

@@ -549,6 +549,10 @@ end
         @test mul!(copy(C), A', A, true, 2) ≈ 3C
         D = Matrix(Hermitian(A * A'))
         @test mul!(copy(D), A, A', true, 3) ≈ 4D
+        if T <: Complex
+            @test mul!(2C, A', A, im, 2) ≈ (4 + im) * C
+            @test mul!(2D, A, A', im, 3) ≈ (6 + im) * D
+        end
     end
 end