decompressing files tries to modify a /*stdout*\ file

[vapier]

Describe the bug
After upgrading to zstd-1.5.1, decompressing files with zstd -dfc shows it trying to modify a /*stdout*\ file. zstd-1.5.0 does not do this.

To Reproduce
Steps to reproduce the behavior:

1. git clone zstd repo
2. run MOREFLAGS=-static make to build it
3. run strace -eutimensat ./programs/zstd -kdfc deutex-5.2.2.tar.zst >/dev/null

utimensat(AT_FDCWD, "/*stdout*\\", [UTIME_NOW, {tv_sec=1609297863, tv_nsec=0} /* 2020-12-29T22:11:03-0500 */], 0) = -1 ENOENT (No such file or directory)

Bisecting the v1.5.0 & v1.5.1 tags says commit 9cd6c1f is to blame.

[vapier]

@felixhandte as the author of said commit