webaccess: fix crash when trying to update closed connection

Webaccess does not handle well disconnections...

Steps for crashing : (in linux at least)
- Be in operate mode in QLC+
- Run firefox to 127.0.0.1:9999
- See a virtual console in firefox, be happy
- Close firefox
- In QLC+ virtual console, move a slider
-> crash

Here we handle disconnection by setting m_conn to NULL in webaccess.