clean up the part where dissect_websocket() is called recursively

Change-Id: Ia94ddb2c4cb03fe5d1af9f8d316e1cdb18fe8283
Reviewed-on: https://code.wireshark.org/review/6143
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>