cmsis-dap: Fix CMSIS-DAP serial number processing.

Check CMSIS-DAP adapter has serialnumber before pass it to wcscmp.
Keep looking for onother adapter if choosed one doesn't have correct
serialnumber.

Change-Id: I7d386a03cb49b9baf22073ae1c6b14269ed3b618
Signed-off-by: Andrii Anpilogov <anpilog@gmail.com>
Reviewed-on: http://openocd.zylin.com/3226
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/cmsis_dap_usb.c

@@ -214,12 +214,14 @@ static int cmsis_dap_usb_open(void)
 			/* we have found an adapter, so exit further checks */
 			/* check serial number matches if given */
 			if (cmsis_dap_serial != NULL) {
-				if (wcscmp(cmsis_dap_serial, cur_dev->serial_number) == 0) {
+				if ((cur_dev->serial_number != NULL) && wcscmp(cmsis_dap_serial, cur_dev->serial_number) == 0) {
 					serial_found = true;
 					break;
 				}
 			} else
 				break;
+
+			found = false;
 		}
 
 		cur_dev = cur_dev->next;