_libssh2_channel_write: handle window_size == 0 better

When about to send data on the channel and the window size is 0, we must
not just return 0 if the transport_read() function returned EAGAIN as it
then causes a busy-loop.

Bug: http://libssh2.org/mail/libssh2-devel-archive-2011-08/0011.shtml

src/channel.c

@@ -2022,7 +2022,7 @@ _libssh2_channel_write(LIBSSH2_CHANNEL *channel, int stream_id,
 
         if(channel->local.window_size <= 0)
             /* there's no room for data so we stop */
-            return 0;
+            return (rc==LIBSSH2_ERROR_EAGAIN?rc:0);
 
         channel->write_bufwrite = buflen;