d3dx9: Fix D3DXMatrixStack_MultMatrix to make tests pass in Windows.

dlls/d3dx9_36/math.c

@@ -855,7 +855,6 @@ static HRESULT WINAPI ID3DXMatrixStackImpl_MultMatrix(ID3DXMatrixStack *iface, C
 
     TRACE("iface %p\n", iface);
 
-    if (!pm) return D3DERR_INVALIDCALL;
     D3DXMatrixMultiply(&This->stack[This->current], &This->stack[This->current], pm);
 
     return D3D_OK;

dlls/d3dx9_36/tests/math.c

@@ -1519,9 +1519,6 @@ static void test_matrix_stack(void)
     ok(SUCCEEDED(hr), "Pop failed, hr %#x\n", hr);
     ok(D3DXMatrixIsIdentity(ID3DXMatrixStack_GetTop(stack)), "The top should be an identity matrix\n");
 
-    hr = ID3DXMatrixStack_MultMatrix(stack, NULL);
-    ok(hr == D3DERR_INVALIDCALL, "Expected D3DERR_INVALIDCALL, got %#x\n", hr);
-
     hr = ID3DXMatrixStack_MultMatrixLocal(stack, NULL);
     ok(hr == D3DERR_INVALIDCALL, "Expected D3DERR_INVALIDCALL, got %#x\n", hr);