morph: fix nil pointer dereferencing to connection

When the RPC connection is lost and a recovery attempt occurs, the variable
`conn` responsible for the connection may be nil and therefore additional
verification is needed.

Closes #3061.

Signed-off-by: Andrey Butusov <[email protected]>

CHANGELOG.md

@@ -9,6 +9,7 @@ Changelog for NeoFS Node
 
 ### Fixed
 - `neofs-cli object delete` command output (#3056)
+- Panic during shutdown if N3 client connection is lost (#3073)
 
 ### Changed
 - Number of cuncurrenly handled notifications from the chain was increased from 10 to 300 for IR (#3068)

pkg/morph/client/multi.go

@@ -64,5 +64,7 @@ func (c *Client) closeWaiter() {
 	case <-c.closeChan:
 	}
 	var conn = c.conn.Swap(nil)
-	conn.Close()
+	if conn != nil {
+		conn.Close()
+	}
 }

pkg/morph/client/notifications.go

@@ -229,6 +229,9 @@ routeloop:
 		if conn == nil {
 			c.logger.Info("RPC connection lost, attempting reconnect")
 			conn = c.switchRPC()
+			if conn == nil {
+				break routeloop
+			}
 			go c.restoreSubscriptions(conn, restoreCh)
 		}
 		var connLost bool