shader: ignore empty color mask in emit_color_select()

Fixes piglit interpolation-flat-other-flat-none.shader_test

src/vrend_shader.c

@@ -679,7 +679,7 @@ static int emit_color_select(struct dump_ctx *ctx)
    char buf[255];
    char *sret = NULL;
 
-   if (!ctx->key->color_two_side)
+   if (!ctx->key->color_two_side || !(ctx->color_in_mask & 0x3))
       return 0;
 
    if (ctx->color_in_mask & 1) {