[gnome-control-center] wifi: Don't access potentially invalid pointer

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit fcf4e44ebddc1a8792f385cc6acfd5556d782375
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Sat Aug 12 18:58:10 2017 -0300

    wifi: Don't access potentially invalid pointer
    
    When the user launches Settings with a panel passed
    as argument, the following things happen:
    
     1. The Wi-Fi panel starts loading.
     2. The command line arguments are passed and the given
        panel is activated.
     3. The Wi-Fi panel cancels the loading routine, and
        rfkill_proxy_acquired_cb() is called with the GError
        set as G_IO_ERROR_CANCELLED.
     4. Crash in rfkill_proxy_acquired_cb().
    
    The crash is caused because, when rfkill_proxy_acquired_cb()
    is called, the CcWifiPanel instance isn't valid anymore. And
    yet, the code tries to cast 'gpointer user_data' to a
    CcWifiPanel pointer.
    
    Fix that by only trying to cast anything after parsing the
    GError set by the callback.

 panels/network/cc-wifi-panel.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)
---
diff --git a/panels/network/cc-wifi-panel.c b/panels/network/cc-wifi-panel.c
index f499938..18ba1ba 100644
--- a/panels/network/cc-wifi-panel.c
+++ b/panels/network/cc-wifi-panel.c
@@ -404,10 +404,12 @@ rfkill_proxy_acquired_cb (GObject      *source_object,
                           GAsyncResult *res,
                           gpointer      user_data)
 {
-  CcWifiPanel *self = CC_WIFI_PANEL (user_data);
-  GError *error = NULL;
+  CcWifiPanel *self;
+  GDBusProxy *proxy;
+  GError *error;
 
-  self->rfkill_proxy = g_dbus_proxy_new_for_bus_finish (res, &error);
+  error = NULL;
+  proxy = g_dbus_proxy_new_for_bus_finish (res, &error);
 
   if (error)
     {
@@ -418,6 +420,10 @@ rfkill_proxy_acquired_cb (GObject      *source_object,
       return;
     }
 
+  self = CC_WIFI_PANEL (user_data);
+
+  self->rfkill_proxy = proxy;
+
   sync_airplane_mode_switch (self);
 }