[v3] fix scoped_allocator problems

[Jonathan Wakely]

        * include/std/scoped_allocator (scoped_allocator::__outermost): Do
        not pass non-POD to varargs function.
        * testsuite/20_util/scoped_allocator/1.cc: Fix test.

This fixes a potential problem in <scoped_allocator> and fixes a
broken test (and ensures it actually runs!)

Tested x86_64-linux, committed to trunk, will put it on 4.7 too.