contrib: pick: bugfix for pick splitting the window excessively

Previously if you carried on past the last message in a pick view pick
would get confused and `forget' about the split pane and would try and
re-split when moving up again. This was due to faulty logic in
notmuch-pick-show-message: something that should have been in the (when message)
clause was not.

Thanks to jrollins for the bug report.