kerneldevice: fix segfault when processing event for non-existing port

    ModemManager[6954]: <warn>  [1618260091.319919] [br0] invalid sysfs path read for net/br0
    **
    ERROR:kerneldevice/mm-kernel-device-generic.c:235:ptr_array_add_sysfs_attribute_link_basename: assertion failed: (array && sysfs_path && attribute)
    Bail out! ERROR:kerneldevice/mm-kernel-device-generic.c:235:ptr_array_add_sysfs_attribute_link_basename: assertion failed: (array && sysfs_path && attribute)

    Thread 1 "ModemManager" received signal SIGABRT, Aborted.
    (gdb) bt
    #0  0x00007ffff7438ef5 in raise () at /usr/lib/libc.so.6
    #1  0x00007ffff7422862 in abort () at /usr/lib/libc.so.6
    #2  0x00007ffff76c2084 in  () at /usr/lib/libglib-2.0.so.0
    #3  0x00007ffff771d85d in g_assertion_message_expr () at /usr/lib/libglib-2.0.so.0
    #4  0x00005555556a5337 in ptr_array_add_sysfs_attribute_link_basename (array=0x5555557de520, sysfs_path=0x0, attribute=0x555555703bd8 "driver", out_value=0x0)
        at kerneldevice/mm-kernel-device-generic.c:235
    #5  0x00005555556a542b in preload_contents_other (self=0x5555557a6c60) at kerneldevice/mm-kernel-device-generic.c:255
    #6  0x00005555556a6485 in preload_contents (self=0x5555557a6c60) at kerneldevice/mm-kernel-device-generic.c:517
    #7  0x00005555556a7fb6 in check_preload (self=0x5555557a6c60) at kerneldevice/mm-kernel-device-generic.c:980
    #8  0x00005555556a855f in initable_init (initable=0x5555557a6c60, cancellable=0x0, error=0x7fffffffe5c8) at kerneldevice/mm-kernel-device-generic.c:1127
    #9  0x00007ffff78a2f41 in g_initable_new_valist () at /usr/lib/libgio-2.0.so.0
    #10 0x00007ffff78a2ffa in g_initable_new () at /usr/lib/libgio-2.0.so.0
    #11 0x00005555556a8205 in mm_kernel_device_generic_new_with_rules (props=0x5555557a5fa0, rules=0x7fffec001e30, error=0x7fffffffe5c8) at kerneldevice/mm-kernel-device-generic.c:1042
    #12 0x00005555556a827d in mm_kernel_device_generic_new (props=0x5555557a5fa0, error=0x7fffffffe5c8) at kerneldevice/mm-kernel-device-generic.c:1063
    #13 0x00005555555aba9c in handle_kernel_event (self=0x555555793220, properties=0x5555557a5fa0, error=0x7fffffffe5c8) at mm-base-manager.c:414