[Bug middle-end/83239] False positive from -Wstringop-overflow on simple std::vector code

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83239

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It was pointed out to me that the allocator in my example should not allocate
more than its max_size, so the example isn't reasonable. So the invariant in
Martin's patch is OK.