Fix a few issues reported by static analyzers

src/irc/irc_channel.cpp

@@ -12,7 +12,7 @@ void IrcChannel::set_self(const std::string& name)
 }
 
 IrcUser* IrcChannel::add_user(const std::string& name,
-                              const std::map<char, char> prefix_to_mode)
+                              const std::map<char, char>& prefix_to_mode)
 {
   this->users.emplace_back(std::make_unique<IrcUser>(name, prefix_to_mode));
   return this->users.back().get();