fix Issue 13446 - Can't use executeShell/escapeShellFileName to redirect to file whose name starts with &

https://issues.dlang.org/show_bug.cgi?id=13446
Looks like this is breaking the Win64 testers, as this can throw a UTF exception. Better test it by hand I guess...
```D
if (filename.length && filename[0] == '&')
   ...
```