[geary] Add a missing cancellable to an engine call in the ConversationMonitor.

[Michael Gratton <mjog src gnome org>]

commit 8dc2222b28ff265cedea4d7d94c44d689300c050
Author: Michael James Gratton <mike vee net>
Date:   Thu Aug 2 20:14:05 2018 +1000

    Add a missing cancellable to an engine call in the ConversationMonitor.
    
    This may work around a crash reported on the mailing list:
    https://mail.gnome.org/archives/geary-list/2018-August/msg00009.html

 src/engine/app/app-conversation-monitor.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/engine/app/app-conversation-monitor.vala b/src/engine/app/app-conversation-monitor.vala
index e3d58de4..eacb9f6e 100644
--- a/src/engine/app/app-conversation-monitor.vala
+++ b/src/engine/app/app-conversation-monitor.vala
@@ -711,7 +711,8 @@ public class Geary.App.ConversationMonitor : BaseObject {
             // Get known paths for all emails
             Gee.MultiMap<Geary.EmailIdentifier, Geary.FolderPath>? email_paths =
                 yield this.base_folder.account.get_containing_folders_async(
-                    job.emails.keys, null
+                    job.emails.keys,
+                    this.operation_cancellable
                 );
 
             // Add them to the conversation set