Fix crash on server shutdown
When AkonadiServer::quit() is called it clears the vector of existing connections and destroys them. This can race with a resource disconnecting or crashing on its own, which then enqueues a singal emission from the Connection to remove itself from the connections vector as well.
Previously the code assumed the Connection must always exist in the vector which caused a crash when it did not...surprise surprise.
BUG: 462692 FIXED-IN: 23.04.0