std.format.internal.write: Segfault in std.utf.decode formatting union

[ibuclaw]

Similar to https://issues.dlang.org/show_bug.cgi?id=7230 (PR #383)

The main difference is that the union has a struct in it.

struct Bug7230Variant
{
    union {
        struct {
            int a;
            int b;
        }
        string c;
    }
}

auto s = Bug7230Variant(b:123);
writeln(s);