[Bug libstdc++/81338] stringstream remains empty after being moved into multiple times

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81338

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This was fixed for 5.5, 6.5, 7.3 and 8.1