(do_exc_finish_channel_handler): Check

channel_count to determine if there are channels left. The code
used to just check next_channel, which was broken.
(alloc_channel): Update channel_count.
(dealloc_channel): Likewise.
(do_exc_finish_channel_handler): The handler for
EXC_FINISH_PENDING shouldn't raise EXC_FINISH_READ.

Rev: src/channel.c:1.92