Fix using pmr::polymorphic_allocator in pre-main

[georgthegreat]

A similar problems for the above (i. e. non-atomic) branch was addressed in #148, yet the default C++11 branch causes segfault with the following stacktrace:

(gdb) bt
#0  0x000000000e1c3d28 in boost::container::pmr::memory_resource::allocate (this=0x0, bytes=92, alignment=4)
    at boost/container/include/boost/container/pmr/memory_resource.hpp:48
#1  0x000000000e2e9e59 in boost::container::pmr::polymorphic_allocator<float>::allocate (this=0x7fffffffd148, n=23)
    at boost/container/include/boost/container/pmr/polymorphic_allocator.hpp:92
#2  0x000000000e2e9cdd in std::__y1::allocator_traits<boost::container::pmr::polymorphic_allocator<float> >::allocate (__a=..., __n=23)
    at libcxx/include/__memory/allocator_traits.h:262
#3  0x000000000e2e9b13 in std::__y1::vector<float, boost::container::pmr::polymorphic_allocator<float> >::__vallocate (this=0x7fffffffd130, __n=23)
    at libcxx/include/vector:958
#4  0x000000000e33c591 in std::__y1::vector<float, boost::container::pmr::polymorphic_allocator<float> >::vector (this=0x7fffffffd130, __n=23)
    at libcxx/include/vector:1106

[igaztanaga]

Many thanks for the patch!