polkitbackend: fix a memory leak when querying NoNewPrivileges - #653

There's currently a memory leak when querying systemd's NoNewPrivileges
property over D-Bus. Specifically, this line:

  g_variant_get (no_new_privs_result, "(v)", &no_new_privs_value);

has one "unfortunate" side-effect - the variant from the "v" specifier
is actually a new reference that has to be freed by the caller, as
stated in GLib's manual:

  Upon encountering a v, g_variant_get() takes a pointer to a (GVariant
  *) (ie: (GVariant **)). It is set to a new reference to a GVariant
  instance containing the contents of the variant value. It is
  appropriate to free this reference using g_variant_unref().

See: https://docs.gtk.org/glib/gvariant-format-strings.html#variants

So, every time this method was called, which can be quite often on
systemd-based desktops, a small GVariant was left behind that could
over time accumulate into a significant memory leak.

The memory leak can be triggered by a following D-Bus call:

busctl call org.freedesktop.PolicyKit1 /org/freedesktop/PolicyKit1/Authority \
            org.freedesktop.PolicyKit1.Authority CheckAuthorization '(sa{sv})sa{ss}us' \
            system-bus-name 1 name s :1.1 org.freedesktop.policykit.exec 0 0 ''

And with Polkit compiled with ASan (and appropriate ASan-aware systemd
unit magic) a single call yields this stack trace:

Direct leak of 65 byte(s) in 1 object(s) allocated from:
    #0 0x7fa8fa4ef41f in malloc (/lib64/libasan.so.8+0xef41f) (BuildId: 80bfc4ae44fdec6ef5fecfb01e2b57d28660991c)
    #1 0x7fa8fa0944b9 in g_malloc (/lib64/libglib-2.0.so.0+0x454b9) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    #2 0x7fa8fa0ea7b1 in g_variant_new_preallocated_trusted (/lib64/libglib-2.0.so.0+0x9b7b1) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    #3 0x7fa8fa0db833 in g_variant_new_boolean (/lib64/libglib-2.0.so.0+0x8c833) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    #4 0x7fa8fa300f4a in parse_value_from_blob.isra.0 (/lib64/libgio-2.0.so.0+0xeef4a) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    #5 0x7fa8fa3017bf in parse_value_from_blob.isra.0 (/lib64/libgio-2.0.so.0+0xef7bf) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    #6 0x7fa8fa30152c in parse_value_from_blob.isra.0 (/lib64/libgio-2.0.so.0+0xef52c) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    #7 0x7fa8fa3063b4 in g_dbus_message_new_from_blob (/lib64/libgio-2.0.so.0+0xf43b4) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    #8 0x7fa8fa3140c9 in _g_dbus_worker_do_read_cb (/lib64/libgio-2.0.so.0+0x1020c9) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    polkit-org#9 0x7fa8fa29685b in g_task_return_now (/lib64/libgio-2.0.so.0+0x8485b) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    polkit-org#10 0x7fa8fa296894 in complete_in_idle_cb (/lib64/libgio-2.0.so.0+0x84894) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    polkit-org#11 0x7fa8fa094463 in g_idle_dispatch (/lib64/libglib-2.0.so.0+0x45463) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    polkit-org#12 0x7fa8fa092e83 in g_main_context_dispatch_unlocked.lto_priv.0 (/lib64/libglib-2.0.so.0+0x43e83) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    polkit-org#13 0x7fa8fa096f77 in g_main_context_iterate_unlocked.isra.0 (/lib64/libglib-2.0.so.0+0x47f77) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    polkit-org#14 0x7fa8fa097226 in g_main_loop_run (/lib64/libglib-2.0.so.0+0x48226) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    polkit-org#15 0x7fa8fa3096c1 in gdbus_shared_thread_func.lto_priv.0 (/lib64/libgio-2.0.so.0+0xf76c1) (BuildId: fe6e790dadb8d9bc9a597e99d3a1bca974179bf0)
    polkit-org#16 0x7fa8fa0cca51 in g_thread_proxy (/lib64/libglib-2.0.so.0+0x7da51) (BuildId: dc61914957dd0fe121ad2089adaf15c832526b63)
    polkit-org#17 0x7fa8fa42b606 in asan_thread_start(void*) (/lib64/libasan.so.8+0x2b606) (BuildId: 80bfc4ae44fdec6ef5fecfb01e2b57d28660991c)

SUMMARY: AddressSanitizer: 65 byte(s) leaked in 1 allocation(s).

The fix is simple - just unref the GVariant once we're done with it.

Also, fix a typo in the variable name (no_new_privis_value ->
no_new_privs_value).

Follow-up for a23d9ce.