Qt4: fix invalid pointer on unknown V4L2 control type (fix #5391)

var_Change(VLC_VAR_GETTEXT) fails if there is no text. In that case,
the value is undefined.

(not cherry picked from commit 9551501fcb7089342c36395cb54a387594e73601)

modules/gui/qt4/components/extended_panels.cpp

@@ -689,7 +689,10 @@ void ExtV4l2::Refresh( void )
         for( int i = 0; i < val.p_list->i_count; i++ )
         {
             const char *psz_var = text.p_list->p_values[i].psz_string;
-            var_Change( p_obj, psz_var, VLC_VAR_GETTEXT, &name, NULL );
+
+            if( var_Change( p_obj, psz_var, VLC_VAR_GETTEXT, &name, NULL ) )
+                continue;
+
             const char *psz_label = name.psz_string;
             msg_Dbg( p_intf, "v4l2 control \"%x\": %s (%s)",
                      val.p_list->p_values[i].i_int, psz_var, name.psz_string );