Why I cannot take a alias to a std::vector element?

On the slide 6 at Rust for C++ programmers, there is this code:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int main()
{
    vector<string> v;
    v.push_back("Hello");

    string& x = v[0];
    v.push_back("world");

    cout << x << endl;
    return 0;
}

Running it I got:

g++ --std=c++11 main.cpp -I . -o main
./main
P▒▒o▒Y ▒▒2.▒8/.▒H/.▒H/.▒X/.▒X/.▒h/.▒h/.▒x/.▒x/.▒▒/.
@▒▒
...

And it keeps going for much more stuff. I found some question about aliases and vectors as:

1. int vs const int&
2. Changing things from a vector

But I could not figure out why the alias is not working based on them. I looked over the http://ift.tt/1nn2SlX, about the vector definition, however it does just seem to be continue memory allocated on the disk. I understand the string Hello and world are allocated somewhere on the data member of the program, as on the assembly here by g++ main.cpp -S:

...
.lcomm _ZStL8__ioinit,1,1
    .def    __main; .scl    2;  .type   32; .endef
    .section .rdata,"dr"
.LC0:
    .ascii "Hello\0"
.LC1:
    .ascii "world\0"
    .text
    .globl  main
    .def    main;   .scl    2;  .type   32; .endef
    .seh_proc   main
main:
...

If I do not push the second element world, the program correctly runs. Therefore why the alias is loosing the reference to the first vector element after the second push?