Take care of null in Wifi setup.

The preference isn't available in Wifi setup launched from
SetupWizard, causing NPE.

Bug: 3404998
Change-Id: Ia0dabea95e5e7424698a269c0851d610fd305309

src/com/android/settings/wifi/WifiSettings.java

@@ -210,11 +210,13 @@ public class WifiSettings extends SettingsPreferenceFragment
             mAddNetwork = findPreference("add_network");
 
             ListPreference pref = (ListPreference) findPreference(KEY_SLEEP_POLICY);
+            if (pref != null) {
                 pref.setOnPreferenceChangeListener(this);
                 int value = Settings.System.getInt(getContentResolver(),
                         Settings.System.WIFI_SLEEP_POLICY,
                         Settings.System.WIFI_SLEEP_POLICY_NEVER);
                 pref.setValue(String.valueOf(value));
+            }
 
             registerForContextMenu(getListView());
             setHasOptionsMenu(true);