2.5.4 stable - NilClass is being extended when it should not be

I'm not sure whether this is an issue with the Treetop grammar or the Treetop parser itself.  I tried to dig in a little but unfortunately I don't know enough about Treetop to fully make sense of what's going on here.

From lib/mail/parsers/rfc2822.rb, lines 2882-2893:
```ruby
      i0 = index
      r1 = _nt_group
      r1.extend(Address0)
      if r1
        r0 = r1
      else
        r2 = _nt_mailbox
        if r2
          r0 = r2
        else
          @index = i0
          r0 = nil
        end
      end
```

The `r1` variable extends `Address0` before checking for truthiness, which results in `NilClass` being extended.