fix crash when mapping unified mailboxes to collections

Summary:
when the fetched unified mailbox collections and loaded mailboxes differ then
trying to get a loaded mailbox using the collection name may fail.
wrapping the call in a try-catch block fixes the crash

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19323

agents/unifiedmailboxagent/unifiedmailboxmanager.cpp

@@ -388,8 +388,12 @@ void UnifiedMailboxManager::discoverBoxCollections(FinishedCallback &&finishedCb
             if (!isUnifiedMailbox(col) || col.parentCollection() == Akonadi::Collection::root()) {
                 continue;
             }
-
-            mMailboxes.at(col.name())->setCollectionId(col.id());
+            const auto it = mMailboxes.find(col.name());
+            if (it == mMailboxes.end()) {
+                qCWarning(UNIFIEDMAILBOXAGENT_LOG) << "Failed to find an unified mailbox for source collection" << col.id();
+            } else {
+                it->second->setCollectionId(col.id());
+            }
         }
     });
     if (finishedCb) {