Minor fix in STOMP broker relay

This is a backport for a change made as part of:
113fd1180a

Issue: SPR-11884

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandler.java

@@ -648,6 +648,8 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 			}
 			finally {
 				try {
+					// Prevent clearConnection() from trying to close
+					this.tcpConnection = null;
 					clearConnection();
 				}
 				catch (Throwable t) {