Don't call buffer_set in im_marked if buffer is None
Don't call buffer_set in im_marked if buffer is None

process_im_marked may be called after process_im_close (e.g. when you do
/close in an im window). After process_im_close has run,
channel.channel_buffer is None, which makes the buffer_set fail.

This fixes #138 and #143.

rawdigits/wee-slack,wee-slack/wee-slack,trygveaa/wee-slack,scharfie/wee-slack,megalithic/wee-slack