Ensure increasing page range list

src/rangelistchecker.cpp

@@ -33,9 +33,15 @@ QJsonArray RangeListChecker::parse(QString str) const
     PrintParameters params;
     params.setPageRange(str.toStdString());
     QJsonArray ret;
+    size_t previous = 0;
     for(const std::pair<size_t, size_t>& p : params.pageRangeList)
     {
+        if(p.first <= previous)
+        {
+            return {};
+        }
         ret.append(QJsonObject {{"low", int(p.first)}, {"high", int(p.second)}});
+        previous = p.second;
     }
     return ret;
 }