Adding null check to avoid abort when invalid IL is encountered

[bholmes]

When a callvirt instruction is encountered on a static method, there is
a hard abort/crash. This commit avoids the problem.

[bholmes]

Here is a small program that shows the abort.

using System;
using System.Reflection;
using System.Reflection.Emit;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            MethodInfo testMethod = typeof(Program).GetMethod("TestMethod");

            DynamicMethod dynamicMethod = new DynamicMethod("TestMethod", typeof(int), new Type[0], restrictedSkipVisibility: true);
            ILGenerator iLGenerator = dynamicMethod.GetILGenerator();
            iLGenerator.Emit(OpCodes.Callvirt, testMethod);
            iLGenerator.Emit(OpCodes.Ret);
            var ttt = (Func<int>)dynamicMethod.CreateDelegate(typeof(Func<int>));

            Console.WriteLine($"{ttt()}");
        }

        static public int TestMethod ()
        {
            return 42;
        }
    }
}

From this sample, the real problem is probably that CreateDelegate is not throwing because of the invalid IL. My commit does not address this aspect.

[bholmes]

Can we sync this with the Mono repo as well?

[lambdageek]

/azp run sync-runtime-to-mono

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[lambdageek]

@bholmes looks like the bot couldn't do the backport to mono/mono for some reason - could you open a PR manually

[bholmes]

@bholmes looks like the bot couldn't do the backport to mono/mono for some reason - could you open a PR manually

mono/mono#21194

[akoeplinger]

The wasm test failures are caused by #57501.

[srxqds]

backport to .net6.0?

[akoeplinger]

/backport to release/6.0-rc1

[github-actions]

Started backporting to release/6.0-rc1: https://github.com/dotnet/runtime/actions/runs/1147170411