comedi: fix divide-by-zero in comedi_buf_munge()

comedi: fix divide-by-zero in comedi_buf_munge()

The comedi_buf_munge() function performs a modulo operation
`async->munge_chan %= async->cmd.chanlist_len` without first
checking if chanlist_len is zero. If a user program submits a command with
chanlist_len set to zero, this causes a divide-by-zero error when the device
processes data in the interrupt handler path.

Add a check for zero chanlist_len at the beginning of the
function, similar to the existing checks for !map and
CMDF_RAWDATA flag. When chanlist_len is zero, update
munge_count and return early, indicating the data was
handled without munging.

This prevents potential kernel panics from malformed user commands.

Reported-by: syzbot+f6c3c066162d2c43a66c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f6c3c066162d2c43a66c
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20250924102639.1256191-1-kartikey406@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>