Qt: fix a crash when the option-list finished by 0, like the new x264_nal_nrd one

(cherry picked from commit e6f9c81589c900980cec11938ed8ae563c78b314)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/preferences_widgets.cpp

@@ -516,6 +516,8 @@ void StringListConfigControl::finish(module_config_t *p_module_config, bool byca
 
     for( int i_index = 0; i_index < p_module_config->i_list; i_index++ )
     {
+        if( !p_module_config->ppsz_list[i_index] )
+              continue;
         combo->addItem( qfu((p_module_config->ppsz_list_text &&
                             p_module_config->ppsz_list_text[i_index])?
                             p_module_config->ppsz_list_text[i_index] :