Fix NULL pointer dereference in archive_acl_from_text_w()

Fix NULL pointer dereference in archive_acl_from_text_w()

When parsing a short "default" ACL prefix (e.g. L"d") with no
subsequent tag field, field[n] is left as {NULL, NULL} and the
code dereferences it unconditionally in the switch statement,
causing a SEGV.

Add a zero-length check after computing the field length so that
malformed entries are skipped with ARCHIVE_WARN, matching the
documented contract. Also move the st pointer computation after
the guard to avoid dereferencing a NULL start pointer.

Fixes libarchive/libarchive#2744