mod_dav: Fix error handling for dav_fs_dir_file_name():

mod_dav: Fix error handling for dav_fs_dir_file_name():

dav_fs_dir_file_name() will not set *fname_p to NULL on failure,
and all callers of dav_fs_dir_file_name() does not check the
return value of dav_fs_dir_file_name(), which could lead to an
undefined behavior against fname_p.

Fix this by adding return value check of dav_fs_dir_file_name()

Submitted by: Zhou Qingyang <zhou1615 umn.edu>
Github: closes #309

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1923813 13f79535-47bb-0310-9956-ffa450edef68