Fix color @usernames sometimes not working at all (#3170)

Definitely memory fuckery involved - The comment from @lubieerror https://github.com/Chatterino/chatterino2/issues/2822#issuecomment-897252673 is finally what led me to adding tests and hopefully fixing this.

src/common/ChannelChatters.cpp

@@ -74,6 +74,12 @@ void ChannelChatters::updateOnlineChatters(
     chatters_->updateOnlineChatters(chatters);
 }
 
+size_t ChannelChatters::colorsSize() const
+{
+    auto size = this->chatterColors_.access()->size();
+    return size;
+}
+
 const QColor ChannelChatters::getUserColor(const QString &user)
 {
     const auto chatterColors = this->chatterColors_.access();