registry: Fix crash on docset removal (fixes #420)

According to the bug report sometimes ListModel::removeDocset() is called
for docsets not in the model. This change adds a simple check, but
does not address the real unknown issue.

src/registry/listmodel.cpp

@@ -193,6 +193,10 @@ void ListModel::addDocset(const QString &name)
 void ListModel::removeDocset(const QString &name)
 {
     const int index = m_docsetItems.keys().indexOf(name);
+    /// TODO: Investigate why this can happen (see #420)
+    if (index == -1)
+        return;
+
     beginRemoveRows(QModelIndex(), index, index);
 
     DocsetItem *docsetItem = m_docsetItems.take(name);