Skip 0-length ACL fields

Skip 0-length ACL fields

Currently, it is possible to create an archive that crashes bsdtar
with a malformed ACL:

Program received signal SIGSEGV, Segmentation fault.
archive_acl_from_text_l (acl=<optimised out>, text=0x7e2e92 "", want_type=<optimised out>, sc=<optimised out>) at libarchive/archive_acl.c:1726
1726				switch (*s) {
(gdb) p n
$1 = 1
(gdb) p field[n]
$2 = {start = 0x0, end = 0x0}

Stop this by checking that the length is not zero before beginning
the switch statement.

I am pretty sure this is the bug mentioned in the qsym paper [1],
and I was able to replicate it with a qsym + AFL + afl-rb setup.

[1] https://www.usenix.org/conference/usenixsecurity18/presentation/yun