Add validity checks for mentions before inserting them into the message

For yet unknown reasons, sometimes invalid mentions are inserted into the list, which will mess up all further messages. While finding those bugs is the better approach, these checks should also be useful.

src/actionshandler.cpp

@@ -77,10 +77,14 @@ void ActionsHandler::handleMessage()
     });
 
     for (const auto &mention : *m_room->mentions()) {
+        if (mention.text.isEmpty() || mention.id.isEmpty()) {
+            continue;
+        }
         handledText = handledText.replace(mention.cursor.anchor(),
                                           mention.cursor.position() - mention.cursor.anchor(),
                                           QStringLiteral("[%1](https://matrix.to/#/%2)").arg(mention.text, mention.id));
     }
+    m_room->mentions()->clear();
 
     if (NeoChatConfig::allowQuickEdit()) {
         QRegularExpression sed("^s/([^/]*)/([^/]*)(/g)?$");