RedisConnection: copy callback before calling it

This allows the callback to call RedisConnection::SetConnectedCallback() to set
another callback for this connection. This sets m_ConnectedCallback and thereby
destroys the std::function while it's running resulting in undefined behavior.
By operating on a copy, m_ConnectedCallback can be set without affecting the
currently running callback.

lib/icingadb/redisconnection.cpp

@@ -323,8 +323,10 @@ void RedisConnection::Connect(asio::yield_context& yc)
 
 			Log(m_Parent ? LogNotice : LogInformation, "IcingaDB", "Connected to Redis server");
 
-			if (m_ConnectedCallback) {
-				m_ConnectedCallback(yc);
+			// Operate on a copy so that the callback can set a new callback without destroying itself while running.
+			auto callback (m_ConnectedCallback);
+			if (callback) {
+				callback(yc);
 			}
 
 			break;