QProcess: ensure we don't accidentally execute something from CWD

QProcess: ensure we don't accidentally execute something from CWD

Origin: upstream, https://download.qt.io/official_releases/qt/5.15/CVE-2022-25255-qprocess5-15.diff
Last-Update: 2022-02-21

Unless "." (or the empty string) is in $PATH, we're not supposed to find
executables in the current directory. This is how the Unix shells behave
and we match their behavior. It's also the behavior Qt had prior to 5.9
(commit 28666d167aa8e602c0bea25ebc4d51b55005db13). On Windows, searching
the current directory is the norm, so we keep that behavior.

This commit does not add an explicit check for an empty return from
QStandardPaths::findExecutable(). Instead, we allow that empty string to
go all the way to execve(2), which will fail with ENOENT. We could catch
it early, before fork(2), but why add code for the error case?

See https://kde.org/info/security/advisory-20220131-1.txt

Gbp-Pq: Name CVE-2022-25255.diff