[v3, 3.4] Fix libstdc++/21286

[Gabriel Dos Reis]

Paolo Carlini <pcarlini@suse.de> writes:

| Hi everyone, hi Gaby,
| 
| Ralf Fassel solicitate an action in 3_4-branch too, because this bug is
| actually a regression from 3.4.2. Personally, I consider the fix safe,
| because this code is generally stable and we had the fix in mainline and
| 4_0-branch for some time.
| 
| Ok?

Yes, I defer to your judgment.

-- Gaby