Before the device is setup, we call nm_platform_tun_get_properties() without
a valid ifindex. That triggered an assertion [1].

Thereby, change nm_platform_tun_get_properties() to effectively clear
the tun properties when we are unable to fetch them. Also, never modify
the tun-mode of NMDeviceTun.

[1]
    #0  0x00007f0a4173e81b in g_logv (breakpoint=1) at gmessages.c:324
    #1  0x00007f0a4173e81b in g_logv (log_domain=0x561e9264ccf6 "NetworkManager", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7ffd71a0d4d0) at gmessages.c:1081
    #2  0x00007f0a4173e98f in g_log (log_domain=<optimized out>, log_level=<optimized out>, format=<optimized out>) at gmessages.c:1119
    #3  0x0000561e9241ecec in nm_platform_tun_get_properties (self=0x561e9354ba70 [NMLinuxPlatform], ifindex=0, props=0x7ffd71a0d650) at platform/nm-platform.c:2081
    #4  0x0000561e923aad9c in reload_tun_properties (self=0x561e937fb080 [NMDeviceTun]) at devices/nm-device-tun.c:68
    #5  0x0000561e923aa795 in realize (device=0x561e937fb080 [NMDeviceTun], plink=0x7ffd71a0d818, error=0x7ffd71a0d798) at devices/nm-device-tun.c:225
    #6  0x0000561e923bdc06 in nm_device_realize (self=0x561e937fb080 [NMDeviceTun], plink=0x7ffd71a0d818, out_compatible=0x7ffd71a0d77c, error=0x7ffd71a0d798) at devices/nm-device.c:1713
    #7  0x0000561e924ad995 in platform_link_added (self=0x561e9356e230 [NMManager], ifindex=33, plink=0x7ffd71a0d818) at nm-manager.c:1947
    #8  0x0000561e924ad717 in _platform_link_cb_idle (data=0x561e937eb940) at nm-manager.c:2029
    #9  0x00007f0a41737e3a in g_main_context_dispatch (context=0x561e93547530) at gmain.c:3154
    #10 0x00007f0a41737e3a in g_main_context_dispatch (context=context@entry=0x561e93547530) at gmain.c:3769
    #11 0x00007f0a417381d0 in g_main_context_iterate (context=0x561e93547530, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3840
    #12 0x00007f0a417384f2 in g_main_loop_run (loop=0x561e935475f0) at gmain.c:4034
    #13 0x0000561e923ba3f3 in main (argc=1, argv=0x7ffd71a0dc68) at main.c:488

Fixes: 4dbaac4b