Fix the following crash:
$ nmcli device monitor a
Error: Device 'a' not found.
Segmentation fault (core dumped)
Found by coverity:
1. NetworkManager-1.41.3/src/nmcli/devices.c:0: scope_hint: In function 'do_devices_monitor'
2. NetworkManager-1.41.3/src/nmcli/devices.c:2932:28: warning[-Wanalyzer-null-dereference]: dereference of NULL 'devices'
2930| }
2931|
2932|-> for (i = 0; i < devices->len; i++)
2933| device_watch(nmc, g_ptr_array_index(devices, i));
2934|
Fixes: 2074b28976 ('nmcli/devices: return GPtrArray instead of GSList from get_device_list()')