C++ dereferencing problem

#include <iostream>
#include <list>
using namespace std;


int main() {

    list<int> L1;
    list<int> L2;

    L1.push_back(1);
    L1.push_back(3);
    L1.push_back(8);
    L1.push_back(12);

    L2.push_back(3);
    L2.push_back(4);
    L2.push_back(8);

    listIntersection(L1, L2);

    system("pause");
    return 0;


}



// ALTHOUGH IT'S A TEMPLATE, I'M JUST USING INTS, SO I CAN USE < > etc.
template<typename T>
list<T> listIntersection(const list<T>& L1, const list<T>& L2) {

    if ( L1.empty() && L2.empty() ) {
        cout << "Returning an empty list because the two arugment lists were empty\n\n";
        list<int> L3;
        return L3;
    }


    list<T>::const_iterator itr1 = L1.begin();
    list<T>::const_iterator itr2 = L2.begin();

    // Create New List
    list<int> L3;

    for (itr1; itr1 != L1.end(); itr1++) {

        while ( *itr2 < *itr1 ) {
               itr2++;
        }
        if ( (*itr1 == *itr2) && (itr2 != L2.end()) ) {
            int tmp = *itr1;
            L3.push_back(tmp);
        }
    }
    return L3;
}