[PaulLodge]

Fix for crash on windows

Crash (STATUS_ACCESS_VIOLATION / 0xC0000005) on all DAV write operations targeting
non-existent resources (PUT to new file, COPY/MOVE to new destination, MKCOL):

dav_fs_quota_precondition() in modules/dav/fs/quota.c called dav_fs_fname(src/dst)
to get the filesystem path

dav_fs_fname() returns resource->info->finfo.fname — APR's stat result

On Windows, apr_stat() calls fillin_fileinfo() which does memset(finfo, '\0', ...),
zeroing finfo->fname = NULL when the file doesn't exist

On Linux, apr_stat() sets finfo->fname = fname (the input path) even when stat succeeds,
but for non-existent resources Apache pre-assigns r->finfo.fname from r->filename,
so it is non-NULL

ap_make_dirstr_parent(pool, NULL) → ap_strrchr_c(NULL, '/') → crash

The crash has been verified by hand to ensure that the AI produced a
correct analysis the associated tests pass as expected, both on Linux
which was not affected, and on Windows.

Co-Authored-By: BOB BOB@ibm.com

---

• This PR does not report or address a security vulnerability
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
• Code follows the httpd style guide.
• New log messages (level debug or higher) use an empty APLOGNO() tag;
  numbers are assigned by a committer at merge time (see docs/log-message-tags/README).
• If the change is user-visible, a changes-entries/*.txt file is included,
  following the template in README.CHANGES (not needed otherwise).
• Test cases based on pyhttpd are included where appropriate, in the
  test/modules/xxx directory matching the modules/xxx module source,
  or test/modules/core for core server changes. On Unix, verify these
  pass locally with e.g. make check-pytest PYTEST_DIRS=test/modules/xxx.