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();

src/irc/irc_channel.hpp

@@ -21,7 +21,7 @@ public:
   void set_self(const std::string& name);
   IrcUser* get_self() const;
   IrcUser* add_user(const std::string& name,
-                    const std::map<char, char> prefix_to_mode);
+                    const std::map<char, char>& prefix_to_mode);
   IrcUser* find_user(const std::string& name) const;
   void remove_user(const IrcUser* user);
   void remove_all_users();