[Bug libstdc++/79254] basic_string::operator= isn't exception safe

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79254

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 40604
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40604&action=edit
Untested patch