Commit c1bf3baa44 removed all but one of the
pthread_exit() calls which used to call winClipboardThreadExit()
Fix the final remaining one to exit via done label on IOError instead.
Also fix a comment and report pre-flush failure to log, but do not exit
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>