Preallocated attributes breaks the null-check on indexers

[rynowak]

From aspnet/Mvc#5920 (comment)

The relevant bits:

            __Microsoft_AspNetCore_Mvc_TagHelpers_AnchorTagHelper = CreateTagHelper<global::Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper>();
            __tagHelperExecutionContext.Add(__Microsoft_AspNetCore_Mvc_TagHelpers_AnchorTagHelper);
            __Microsoft_AspNetCore_Mvc_TagHelpers_AnchorTagHelper.RouteValues["page"] = (string)__tagHelperAttribute_0.Value;
            __tagHelperExecutionContext.AddTagHelperAttribute(__tagHelperAttribute_0);
            if (__Microsoft_AspNetCore_Mvc_TagHelpers_AnchorTagHelper.RouteValues == null)
            {
                throw new InvalidOperationException(InvalidTagHelperIndexerAssignment("asp-route-id", "Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper", "RouteValues"));
            }

The RouteValues is accessed before the error check. page in this case is a preallocated attribute.

[ajaybhargavb]

27ac5da