[Bug libstdc++/94906] memory corruption in std::pmr::memory_buffer_resource

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94906

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Better would be:
size_t __size = (((size_t)1) << __ch->_M_size);

As long could be 32bits (windows)