Fix crash after client has been unable to connect to a broker

This is part of the cleanup routine when the client is already exiting.

Closes #1246. Thanks to Christoph Krey.

ChangeLog.txt

@@ -8,6 +8,11 @@ Broker:
   `response-topic`.  Closes #1244.
 - Fix build for WITH_TLS=no. Closes #1250.
 
+Client library:
+- Fix crash after client has been unable to connect to a broker. This occurs
+  when the client is exiting and is part of the final library cleanup routine.
+  Closes #1246.
+
 
 1.6.1 - 20190426
 ================

lib/net_mosq.c

@@ -107,6 +107,7 @@ static void cleanup_ui_method(void)
 {
 	if(_ui_method){
 		UI_destroy_method(_ui_method);
+		_ui_method = NULL;
 	}
 }