support @asyncCall with null as the result pointer

[andrewrk]

This should work:

const std = @import("std");
const expect = std.testing.expect;

test "async fn pointer in a struct field" {
    var data: i32 = 1;
    const Foo = struct {
        bar: async fn (*i32) i32,
    };
    var foo = Foo{ .bar = simpleAsyncFn2 };
    var bytes: [64]u8 = undefined;
    const f = @asyncCall(&bytes, null, foo.bar, &data);
    comptime expect(@typeOf(f) == anyframe->i32);
    expect(data == 2);
    resume f;
    expect(data == 4);
}

async fn simpleAsyncFn2(y: *i32) i32 {
    defer y.* += 2;
    y.* += 1;
    suspend;
    return 1234;
}

Current output:

Segmentation fault at address 0x0