fix null dereference on boards without analog output

diff --git a/comedi/drivers/cb_pcidas64.c b/comedi/drivers/cb_pcidas64.c
index cfbc0c073520c260da3c3c1c087c6b1586fbba37..212627224ab3e56d3d6924ef2b448b17e17ef1a0 100644 (file)
--- a/comedi/drivers/cb_pcidas64.c
+++ b/comedi/drivers/cb_pcidas64.c
@@ -1366,6 +1366,7 @@ static int setup_subdevices(comedi_device *dev)
        } else
        {
                s->type = COMEDI_SUBD_UNUSED;
+               dev->write_subdev = NULL;
        }
 
        // digital input
@@ -2948,13 +2949,14 @@ static void handle_ao_interrupt(comedi_device *dev, unsigned short status,
        unsigned int plx_status)
 {
        comedi_subdevice *s = dev->write_subdev;
-       comedi_async *async = s->async;
-       comedi_cmd *cmd = &async->cmd;
+       comedi_async *async;
+       comedi_cmd *cmd;
        uint8_t dma0_status;
        unsigned long flags;
 
-       /* board might not support ao, in which case async is NULL */
-       if(async == NULL) return;
+       /* board might not support ao, in which case write_subdev is NULL */
+       if(s == NULL) return;
+       async = s->async;
        cmd = &async->cmd;
 
        // spin lock makes sure noone else changes plx dma control reg