maildir-syncback: forward message (with only attachment) fails

The maidir-syncback branch adds a ```send_message``` to forward mode which does not account for when only an attachment has been marked and forwarded. In which case there is no ```@m``` available to add the :forwarded label to.

Possible solution, just skip the label adding if no ```@m``` since its not the original message that has been forwarded anyway...

```
--- NoMethodError from thread: main
undefined method `add_label' for nil:NilClass
/home/gaute/.rvm/gems/ruby-1.9.3-p392/gems/sup-999/lib/sup/modes/forward_mode.rb:75:in `send_message'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/gems/sup-999/lib/sup/mode.rb:59:in `handle_input'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/gems/sup-999/lib/sup/buffer.rb:273:in `handle_input'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/gems/sup-999/bin/sup:265:in `<module:Redwood>'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/gems/sup-999/bin/sup:73:in `<top (required)>'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/bin/sup:19:in `load'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/bin/sup:19:in `<main>'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/bin/ruby_noexec_wrapper:14:in `eval'
/home/gaute/.rvm/gems/ruby-1.9.3-p392/bin/ruby_noexec_wrapper:14:in `<main>'
```