erofs-utils: lib: actually skip the unidentified xattrs

erofs-utils: lib: actually skip the unidentified xattrs

parse_one_xattr() will return null if it detects unidentified xattr.
In such cases we need to skip this xattr which was our intention than
try to add it in erofs_xattr_add() which results in null pointer
dereference.

Fixes: 3037f8958f3b ("erofs-utils: skip all unidentified xattrs from local paths")
Signed-off-by: Sandeep Dhavale <dhavale@google.com>
Link: https://lore.kernel.org/r/20240820210123.2684886-2-dhavale@google.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>