SipVoipLink: prevent a crash at shutdown

SipVoipLink may never be intialized in Manager instance (Manager::init())
leading to a crash when the Manager calls finish(). Added a check
to prevent the crash.

Gitlab: #481

Change-Id: I817a7795ce4341e6214a44e46037b0183fde53c5

src/manager.cpp

@@ -888,8 +888,10 @@ Manager::finish() noexcept
         // NOTE: sipLink_->shutdown() is needed because this will perform
         // sipTransportBroker->shutdown(); which will call Manager::instance().sipVoIPLink()
         // so the pointer MUST NOT be resetted at this point
-        pimpl_->sipLink_->shutdown();
-        pimpl_->sipLink_.reset();
+        if (pimpl_->sipLink_) {
+            pimpl_->sipLink_->shutdown();
+            pimpl_->sipLink_.reset();
+        }
 
         pj_shutdown();