sipaccount: only create weak reference if it didn't exist

Change-Id: Ifedd141b70fdf31f07e4a1b572392fe71d62d589

src/sip/sipaccount.cpp

@@ -961,6 +961,7 @@ SIPAccount::startKeepAliveTimer()
         if (auto sipAccount = static_cast<std::weak_ptr<SIPAccount>*>(te->user_data)->lock())
             sipAccount->keepAliveRegistrationCb();
     };
+    if (not keepAliveTimer_.user_data)
         keepAliveTimer_.user_data = new std::weak_ptr<SIPAccount>(weak());
     keepAliveTimer_.id = timerIdDist_(rand);