qt: fix a couple of leaks in expert prefs

Identified by a different invocation of ASAN than I'd previously used.

wrt. the second fix it seems `clear()` alone does not destroy these items.

```
Direct leak of 64560 byte(s) in 1345 object(s) allocated from:
    #0 0x7f1f87268f37 in operator new(unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f1f71099981 in ExpertPrefsTableModel::ExpertPrefsTableModel(module_t**, unsigned long, QWidget*) ../../modules/gui/qt/dialogs/preferences/expert_model.cpp:260
    #2 0x7f1f710a32fa in PrefsDialog::setExpert() ../../modules/gui/qt/dialogs/preferences/preferences.cpp:106
    #3 0x7f1f710a8851 in PrefsDialog::PrefsDialog(QWindow*, qt_intf_t*) ../../modules/gui/qt/dialogs/preferences/preferences.cpp:74
    #4 0x7f1f70ee43e2 in DialogsProvider::prefsDialog() ../../modules/gui/qt/dialogs/dialogs_provider.cpp:253
    #5 0x7f1f714f6f97 in DialogsProvider::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:268
    #6 0x7f1f714f7c11 in DialogsProvider::qt_metacall(QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:388
    #7 0x7f1f70568f1c  (/lib/x86_64-linux-gnu/libQt5Qml.so.5+0x2c4f1c)

...

Direct leak of 1112 byte(s) in 122 object(s) allocated from:
    #0 0x7f1f872127a7 in __interceptor_strdup ../../../../src/libsanitizer/asan/asan_interceptors.cpp:454
    #1 0x7f1f710950f0 in ExpertPrefsTableItem::ExpertPrefsTableItem(module_config_t*, QString const&, QString const&, bool) ../../modules/gui/qt/dialogs/preferences/expert_model.cpp:61
    #2 0x7f1f710999ac in ExpertPrefsTableModel::ExpertPrefsTableModel(module_t**, unsigned long, QWidget*) ../../modules/gui/qt/dialogs/preferences/expert_model.cpp:260
    #3 0x7f1f710a32fa in PrefsDialog::setExpert() ../../modules/gui/qt/dialogs/preferences/preferences.cpp:106
    #4 0x7f1f710a8851 in PrefsDialog::PrefsDialog(QWindow*, qt_intf_t*) ../../modules/gui/qt/dialogs/preferences/preferences.cpp:74
    #5 0x7f1f70ee43e2 in DialogsProvider::prefsDialog() ../../modules/gui/qt/dialogs/dialogs_provider.cpp:253
    #6 0x7f1f714f6f97 in DialogsProvider::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:268
    #7 0x7f1f714f7c11 in DialogsProvider::qt_metacall(QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:388
    #8 0x7f1f70568f1c  (/lib/x86_64-linux-gnu/libQt5Qml.so.5+0x2c4f1c)

...

Direct leak of 33 byte(s) in 33 object(s) allocated from:
    #0 0x7f1f872677cf in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7f1f86eac342 in vlc_alloc ../../include/vlc_common.h:1149
    #2 0x7f1f86eac342 in module_config_get ../../src/modules/modules.c:346
    #3 0x7f1f710995f5 in ExpertPrefsTableModel::ExpertPrefsTableModel(module_t**, unsigned long, QWidget*) ../../modules/gui/qt/dialogs/preferences/expert_model.cpp:230
    #4 0x7f1f710a32fa in PrefsDialog::setExpert() ../../modules/gui/qt/dialogs/preferences/preferences.cpp:106
    #5 0x7f1f710a8851 in PrefsDialog::PrefsDialog(QWindow*, qt_intf_t*) ../../modules/gui/qt/dialogs/preferences/preferences.cpp:74
    #6 0x7f1f70ee43e2 in DialogsProvider::prefsDialog() ../../modules/gui/qt/dialogs/dialogs_provider.cpp:253
    #7 0x7f1f714f6f97 in DialogsProvider::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:268
    #8 0x7f1f714f7c11 in DialogsProvider::qt_metacall(QMetaObject::Call, int, void**) gui/qt/dialogs/dialogs_provider.moc.cpp:388
    #9 0x7f1f70568f1c  (/lib/x86_64-linux-gnu/libQt5Qml.so.5+0x2c4f1c)
```

modules/gui/qt/dialogs/preferences/expert_model.cpp

@@ -229,7 +229,11 @@ ExpertPrefsTableModel::ExpertPrefsTableModel( module_t **mod_list, size_t mod_co
         unsigned confsize;
         module_config_t *const config = module_config_get( mod, &confsize );
         if( confsize == 0 )
+        {
+            /* If has items but none are visible, we still need to deallocate */
+            module_config_free( config );
             continue;
+        }
         config_sets.append( config );
 
         bool is_core = module_is_main( mod );
@@ -281,7 +285,10 @@ ExpertPrefsTableModel::ExpertPrefsTableModel( module_t **mod_list, size_t mod_co
 
 ExpertPrefsTableModel::~ExpertPrefsTableModel()
 {
-    items.clear(); /* We must destroy the items before releasing the config set */
+    /* We must destroy the items before releasing the config set */
+    foreach ( ExpertPrefsTableItem *item, items )
+        delete item;
+    items.clear();
     foreach ( module_config_t *config_set, config_sets )
         module_config_free( config_set );
 }