Fix crash on account deletion

Calling forgetSensitiveData() on account deletion leads to a timer for
clearQNAMCache() being queued. Then the Account object is deleted. The
Credentials object stays alive for now because it has a deleteLater
deleter.

If the timer calls into a slot on the Credentials object, the _account
pointer will be invalid at this time.

As a workaround, move the target slot to Account - that way it will not
be called as the account object is already destroyed.

However since Account and Credentials are mutually dependent, it would
be much preferable if their lifetimes were linked, avoiding this
category of bugs.

The current behavior was introduced in
d40c56eda5 and I currently don't
understand why - maybe there's another way of dealing with the problem
that existed then.

src/libsync/account.cpp

@@ -391,6 +391,11 @@ void Account::handleInvalidCredentials()
     emit invalidCredentials();
 }
 
+void Account::clearQNAMCache()
+{
+    _am->clearAccessCache();
+}
+
 const Capabilities &Account::capabilities() const
 {
     return _capabilities;

src/libsync/account.h

@@ -197,6 +197,10 @@ public:
     /// Called by network jobs on credential errors, emits invalidCredentials()
     void handleInvalidCredentials();
 
+public slots:
+    /// Used when forgetting credentials
+    void clearQNAMCache();
+
 signals:
     /// Emitted whenever there's network activity
     void propagatorNetworkActivity();

src/libsync/creds/httpcredentials.cpp

@@ -370,16 +370,7 @@ void HttpCredentials::invalidateToken()
     // indirectly) from QNetworkAccessManagerPrivate::authenticationRequired, which itself
     // is a called from a BlockingQueuedConnection from the Qt HTTP thread. And clearing the
     // cache needs to synchronize again with the HTTP thread.
-    QTimer::singleShot(0, this, SLOT(clearQNAMCache()));
-}
-
-void HttpCredentials::clearQNAMCache()
-{
-#if QT_VERSION >= QT_VERSION_CHECK(5, 0, 0)
-    _account->networkAccessManager()->clearAccessCache();
-#else
-    _account->resetNetworkAccessManager();
-#endif
+    QTimer::singleShot(0, _account, SLOT(clearQNAMCache()));
 }
 
 void HttpCredentials::forgetSensitiveData()

src/libsync/creds/httpcredentials.h

@@ -110,7 +110,6 @@ private Q_SLOTS:
     void slotWriteClientCertPEMJobDone(QKeychain::Job *);
     void slotWriteClientKeyPEMJobDone(QKeychain::Job *);
     void slotWriteJobDone(QKeychain::Job *);
-    void clearQNAMCache();
 
 protected:
     QString _user;