[Bug c/66661] incorrect memory access in optimization with flexible array member

[P at draigBrady dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #1 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 35849
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35849&action=edit
summary code (does not reproduce issue)