wwan: T2529: fix validation logic for non existing devices

A non existing device usb0b1.4p1.?? device was not detected, as find_device_file()
returned None which can not be passed into os.path.exists().

Traceback (most recent call last):
  File "/usr/libexec/vyos/conf_mode/interfaces-wirelessmodem.py", line 126, in <module>
    verify(c)
  File "/usr/libexec/vyos/conf_mode/interfaces-wirelessmodem.py", line 60, in verify
    if dev_path is None or os.path.exists(find_device_file(dev_path)):
  File "/usr/lib/python3.7/genericpath.py", line 19, in exists
    os.stat(path)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

1 files changed, 2 insertions, 1 deletions

diff --git a/src/conf_mode/interfaces-wirelessmodem.py b/src/conf_mode/interfaces-wirelessmodem.py
index 7d8110096..2da1f0863 100755
--- a/src/conf_mode/interfaces-wirelessmodem.py
+++ b/src/conf_mode/interfaces-wirelessmodem.py
@@ -56,7 +56,8 @@ def verify(wwan):
 
     # we can not use isfile() here as Linux device files are no regular files
     # thus the check will return False
-    if not os.path.exists(find_device_file(wwan['device'])):
+    dev_path = find_device_file(wwan['device'])
+    if dev_path is None or not os.path.exists(dev_path):
         raise ConfigError('Device "{device}" does not exist'.format(**wwan))
 
     verify_vrf(wwan)