[Bug libstdc++/67214] undefined behaviour in std::num_get::_M_extract_int()

[ryxi at stu dot xidian.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67214

--- Comment #2 from Xi Ruoyao <ryxi at stu dot xidian.edu.cn> ---
Created attachment 40946
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40946&action=edit
patch for this PR

Since GCC 7 is in stage 4, this patch will be committed in GCC 8 stage 1.