Segfault during bootstrap if bootstrap nodes is empty

* * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)

  * frame #0: 0x00000001009627b5 libc++.1.0.dylib`std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::basic_string(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 37

    frame #1: 0x00000001001242e0 test_integration_collections`couchbase::io::mcbp_session::initiate_bootstrap() [inlined] std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >::pair(this=0x0000000000000000, (null)=0x0000000000000000) at utility:326:5

    frame #2: 0x00000001001242d8 test_integration_collections`couchbase::io::mcbp_session::initiate_bootstrap() [inlined] std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >::pair(this=0x0000000000000000, (null)=0x0000000000000000) at utility:326

    frame #3: 0x00000001001242d8 test_integration_collections`couchbase::io::mcbp_session::initiate_bootstrap() [inlined] couchbase::origin::next_address(this=<unavailable>) at origin.hxx:142

    frame #4: 0x00000001001242ca test_integration_collections`couchbase::io::mcbp_session::initiate_bootstrap(this=<unavailable>) at mcbp_session.hxx:658

    frame #5: 0x000000010011f6dc test_integration_collections`couchbase::io::mcbp_session::bootstrap(this=0x000000010100d220, handler=<unavailable>, retry_on_bucket_not_found=<unavailable>)>&&, bool) at mcbp_session.hxx:626:9

    frame #6: 0x000000010010f72a test_integration_collections`void couchbase::cluster::do_open<test::utils::open_cluster(couchbase::cluster&, couchbase::origin const&)::$_0>(this=0x00007ffeefbfac60, handler=0x0000000000000098)::$_0&&) at cluster.hxx:254:19

    frame #7: 0x000000010010d7e9 test_integration_collections`test::utils::open_cluster(couchbase::cluster&, couchbase::origin const&) [inlined] void couchbase::cluster::open<test::utils::open_cluster(couchbase::cluster&, couchbase::origin const&)::$_0>(this=<unavailable>, origin=0x00007ffeefbf9fe0)::$_0&&) at cluster.hxx:76:9

    frame #8: 0x000000010010d623 test_integration_collections`test::utils::open_cluster(cluster=0x00007ffeefbfac60, origin=0x00007ffeefbf9fe0) at integration_shortcuts.cxx:27

    frame #9: 0x000000010014c815 test_integration_collections`test::utils::integration_test_guard::integration_test_guard(this=0x00007ffeefbfac48) at integration_test_guard.cxx:33:5

    frame #10: 0x000000010004664e test_integration_collections`____C_A_T_C_H____T_E_S_T____8() at test_integration_collections.cxx:125:41

    frame #11: 0x0000000100022868 test_integration_collections`Catch::RunContext::invokeActiveTestCase() [inlined] Catch::TestCase::invoke(this=<unavailable>) const at catch.hpp:14160:15

    frame #12: 0x000000010002285c test_integration_collections`Catch::RunContext::invokeActiveTestCase(this=<unavailable>) at catch.hpp:13020

    frame #13: 0x000000010002037f test_integration_collections`Catch::RunContext::runCurrentTest(this=0x00007ffeefbfb508, redirectedCout="", redirectedCerr="") at catch.hpp:12993:17

    frame #14: 0x000000010001f8c3 test_integration_collections`Catch::RunContext::runTest(this=0x00007ffeefbfb508, testCase=<unavailable>) at catch.hpp:12754:13

    frame #15: 0x000000010002679a test_integration_collections`Catch::Session::runInternal() at catch.hpp:13347:45

    frame #16: 0x0000000100026623 test_integration_collections`Catch::Session::runInternal(this=<unavailable>) at catch.hpp:13553

    frame #17: 0x00000001000259f2 test_integration_collections`Catch::Session::run(this=0x00007ffeefbfb790) at catch.hpp:13509:24

    frame #18: 0x000000010004117f test_integration_collections`main [inlined] int Catch::Session::run<char>(this=0x0000000100224068, argc=<unavailable>, argv=<unavailable>) at catch.hpp:13231:30

    frame #19: 0x0000000100041155 test_integration_collections`main(argc=2, argv=0x00007ffeefbfb948) at catch.hpp:17526

    frame #20: 0x00007fff6c75bcc9 libdyld.dylib`start + 1

    frame #21: 0x00007fff6c75bcc9 libdyld.dylib`start + 1