Occasional crash when killing kakoune session

[falbru]

The kakoune session gets killed when exiting the application. Sometimes that results in a crash, where I get the following error log KakouneClient: "socket read failed: Success\ndisconnecting\n.

According to the kakoune documentation, the kill command that is sent to the session is supposed to kill both the server and client:

• "kill[!] [<exit status>]::
  terminate the current session, all the clients as well as the server.
  If specified, the server and clients exit status will be set to <exit status>"

This doesn't really matter, since the application is exiting anyways, but I'd be nice to clean this up.

[falbru]

It happens infrequently enough that I'll postpone it to another milestone