[libsignon-qt] Guard PendingCall against deletion by connected slots

This commit uses QPointer to guard the PendingCall object (and the
QDBusPendingCallWatcher associated with it).

It should not be required, and this commit should be removed once
the root cause (connected slot which delete's the PendingCall via
side-effect) is found and corrected.

libsignon/lib/SignOn/async-dbus-proxy.cpp

@@ -30,6 +30,7 @@
 #include <QDebug>
 #include <QMetaMethod>
 #include <QMetaType>
+#include <QPointer>
 
 #include "connection-manager.h"
 #include "dbusinterface.h"
@@ -117,12 +118,16 @@ void PendingCall::onFinished(QDBusPendingCallWatcher *watcher)
         }
     }
 
+    QPointer<PendingCall> thisguard(this);
+    QPointer<QDBusPendingCallWatcher> watcherguard(watcher);
     if (watcher->isError()) {
         Q_EMIT error(watcher->error());
     } else {
         Q_EMIT success(watcher);
     }
-    Q_EMIT finished(watcher);
+    if (thisguard) {
+        Q_EMIT finished(watcherguard ? watcher : 0);
+    }
 }
 
 void PendingCall::onInterfaceDestroyed()