nqueen example fails occasionally

[msimberg]

The nqueen usually fails after a few times with the following error (with both default and size N commands):

...
{version}: V1.1.0 (AGAS: V3.0), Git: 2ec00bc39b
{boost}: V1.65.0
{build-type}: release
{date}: Dec  5 2017 09:55:05
{platform}: linux
{compiler}: GNU C++ version 6.2.0 20160822 (Cray Inc.)
{stdlib}: GNU libstdc++ version 20160822
{what}: vector::_M_range_check: __n (which is 0) >= this->size() (which is 0): HPX(unhandled_exception)

terminate called after throwing an instance of 'hpx::detail::exception_with_info<hpx::exception>'
  what():  vector::_M_range_check: __n (which is 0) >= this->size() (which is 0): HPX(unhandled_exception)
Aborted