S&R: Fix assert when no result is selected

It is OK to pass an invalid QModelIndex to isMatch(), nextMatch(),
prevMatch()

addons/search/Results.cpp

@@ -80,7 +80,7 @@ bool Results::isEmpty() const
 
 bool Results::isMatch(const QModelIndex &index) const
 {
-    Q_ASSERT(index.model() == model());
+    Q_ASSERT(!index.isValid() || index.model() == model());
     return matchModel.isMatch(model()->mapToSource(index));
 }
 
@@ -101,13 +101,13 @@ QModelIndex Results::firstMatch() const
 
 QModelIndex Results::nextMatch(const QModelIndex &itemIndex) const
 {
-    Q_ASSERT(itemIndex.model() == model());
+    Q_ASSERT(!itemIndex.isValid() || itemIndex.model() == model());
     return model()->mapFromSource(matchModel.nextMatch(model()->mapToSource(itemIndex)));
 }
 
 QModelIndex Results::prevMatch(const QModelIndex &itemIndex) const
 {
-    Q_ASSERT(itemIndex.model() == model());
+    Q_ASSERT(!itemIndex.isValid() || itemIndex.model() == model());
     return model()->mapFromSource(matchModel.prevMatch(model()->mapToSource(itemIndex)));
 }
 

addons/search/plugin_search.cpp

@@ -1342,7 +1342,7 @@ void KatePluginSearchView::replaceSingleMatch()
     }
 
     QModelIndex itemIndex = res->treeView->currentIndex();
-    if (!res->isMatch(itemIndex)) {
+    if (!itemIndex.isValid() || !res->isMatch(itemIndex)) {
         goToNextMatch();
     }