[Bug libstdc++/79820] std::ifstream sets errno to zero

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79820

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The bug title says std::ifstream sets errno to zero, but it should never run
stdio_filebuf::sys_open. Do you have a testcase for this?

We should still fix it even if it only affects a non-standard extension, but
it's probably not worth backporting if it doesn't affect std::ifstream.