[RFA] Adding n <= max_size check in allocator::allocate

[Paolo Carlini]

Hi,

consider this:

  std::vector<int> v(size_t(-1) / sizeof(int) + 1, 1);

currently, instead of throwing (bad_alloc), seg faults.

Usual story: in mt_allocator there are no checks for overflows
when n * sizeof(value_type) is computed to call ::operator new,
and the latter happily allocates only *zero* bytes...

Of course this does *not* happen with pool_allocator::allocate
that wraps everything in:

  if (__n)
    {
      if (__n <= this->max_size())
        {
          ...
        }
      else
        __throw_bad_alloc();
    }

 From a QoI point of view, at least, I think we should have these
checks in our allocators (only pool_allocator is ok!) and mean to
work on this ASAP, together with the corresponding testcases.

Paolo.