[geary] Really fix (?) critical warning when changing conversations rapidly.

[Michael Gratton <mjog src gnome org>]

commit 4a893e47080b039b64736e0e575aeaf097e1eb0f
Author: Michael James Gratton <mike vee net>
Date:   Tue Aug 2 22:32:50 2016 +1000

    Really fix (?) critical warning when changing conversations rapidly.
    
    Commit f6518b0 relied on the load being cancelled, which only happens if
    you change conversation, not folder. Changing the foldler will eventually
    change the conversation, but not after a nnumber of async calls.
    
    Bug 714509.
    
    * src/client/conversation-viewer/conversation-viewer.vala
      (ConversationViewer::on_folder_selected): Cancel existing load before
      preceeding.

 .../conversation-viewer/conversation-viewer.vala   |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/client/conversation-viewer/conversation-viewer.vala 
b/src/client/conversation-viewer/conversation-viewer.vala
index 33a86fb..99fbb58 100644
--- a/src/client/conversation-viewer/conversation-viewer.vala
+++ b/src/client/conversation-viewer/conversation-viewer.vala
@@ -419,6 +419,7 @@ public class ConversationViewer : Gtk.Box {
     
     private void on_folder_selected(Geary.Folder? folder) {
         hide_special_message();
+        cancel_load();
         
         current_folder = folder;
         email_store = (current_folder == null ? null : new Geary.App.EmailStore(current_folder.account));