Fix an undefined behavior of dav_fs_dir_file_name()

[zhou1615]

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()