staging: comedi: verify array index is correct before using it

This code reads from the array before verifying that "trig" is a valid
index.  If the index is wildly out of bounds then reading from an
invalid address could lead to an Oops.

Fixes: a8c66b6 ("staging: comedi: addi_apci_1500: rewrite the subdevice support functions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20200709102936.GA20875@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Dan Carpenter

drivers/staging/comedi/drivers/addi_apci_1500.c

@@ -456,9 +456,9 @@ static int apci1500_di_cfg_trig(struct comedi_device *dev,
 	unsigned int lo_mask = data[5] << shift;
 	unsigned int chan_mask = hi_mask | lo_mask;
 	unsigned int old_mask = (1 << shift) - 1;
-	unsigned int pm = devpriv->pm[trig] & old_mask;
-	unsigned int pt = devpriv->pt[trig] & old_mask;
-	unsigned int pp = devpriv->pp[trig] & old_mask;
+	unsigned int pm;
+	unsigned int pt;
+	unsigned int pp;
 
 	if (trig > 1) {
 		dev_dbg(dev->class_dev,
@@ -471,6 +471,10 @@ static int apci1500_di_cfg_trig(struct comedi_device *dev,
 		return -EINVAL;
 	}
 
+	pm = devpriv->pm[trig] & old_mask;
+	pt = devpriv->pt[trig] & old_mask;
+	pp = devpriv->pp[trig] & old_mask;
+
 	switch (data[2]) {
 	case COMEDI_DIGITAL_TRIG_DISABLE:
 		/* clear trigger configuration */