core: reset screenshot pointer aswell (#209)

ensure the screenshot portal is destructed before the connection is
down, otherwise it segfaults inside sdbus.

src/core/PortalManager.cpp

@@ -482,6 +482,7 @@ void CPortalManager::startEventLoop() {
 
     m_sPortals.globalShortcuts.reset();
     m_sPortals.screencopy.reset();
+    m_sPortals.screenshot.reset();
 
     m_pConnection.reset();
     pw_loop_destroy(m_sPipewire.loop);