mu4e: don't be too eager to open view

When there's no view yet, don't automatically open one when moving.

NEWS.org

@@ -6,6 +6,9 @@
   
    - change the message file names to better conform to the maildir
      spec; this was confusing some tools.
+   - fix navigation when not running in split-view mode
+   - add mu4e-view-body-face, so the body-face for message in the view can be customized; e.g.
+      (set-face-attribute 'mu4e-view-body-face nil :font "Liberation Serif-10")
   
 ** 0.9.13
 

mu4e/mu4e-headers.el

@@ -1448,7 +1448,8 @@ docid. Otherwise, return nil."
       ;; attempt to highlight the new line, display the message
       (mu4e~headers-highlight docid)
       ;; update message view if it was already showing
-      (mu4e-headers-view-message)
+      (when (window-live-p mu4e~headers-view-win)
+	(mu4e-headers-view-message))
       docid)))
 
 (defun mu4e-headers-next (&optional n)