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

In the Linux kernel, the following vulnerability has been resolved: 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.