tmpfiles: avoid null free() for acl attributes

When built with ACL support, we might be processing a tmpfiles
entry where there's no cause for us to call parse_acls_from_arg,
then we get to the end of parse_line without having ever populated
i.{acl_access, acl_default}.

Then we pass a null pointer into acl_free().

From UBSAN w/ GCC 13.0.0_pre20230101:
```
$ systemd-tmpfiles --clean
/var/tmp/portage/sys-apps/acl-2.3.1-r1/work/acl-2.3.1/libacl/acl_free.c:44:14: runtime error: applying non-zero offset 18446744073709551608 to null pointer
    #0 0x7f65d868b482 in acl_free /var/tmp/portage/sys-apps/acl-2.3.1-r1/work/acl-2.3.1/libacl/acl_free.c:44
    systemd#1 0x55fe7e592249 in item_free_contents ../systemd-9999/src/tmpfiles/tmpfiles.c:2855
    systemd#2 0x55fe7e5a347a in parse_line ../systemd-9999/src/tmpfiles/tmpfiles.c:3158
    systemd#3 0x55fe7e5a347a in read_config_file ../systemd-9999/src/tmpfiles/tmpfiles.c:3897
    systemd#4 0x55fe7e590c61 in read_config_files ../systemd-9999/src/tmpfiles/tmpfiles.c:3985
    systemd#5 0x55fe7e590c61 in run ../systemd-9999/src/tmpfiles/tmpfiles.c:4157
    systemd#6 0x55fe7e590c61 in main ../systemd-9999/src/tmpfiles/tmpfiles.c:4218
    systemd#7 0x7f65d7ebe289  (/usr/lib64/libc.so.6+0x23289)
    systemd#8 0x7f65d7ebe344 in __libc_start_main (/usr/lib64/libc.so.6+0x23344)
    systemd#9 0x55fe7e591900 in _start (/usr/bin/systemd-tmpfiles+0x11900)
```

(cherry picked from commit 9f804ab)

src/tmpfiles/tmpfiles.c

@@ -2849,8 +2849,11 @@ static void item_free_contents(Item *i) {
         strv_free(i->xattrs);
 
 #if HAVE_ACL
-        acl_free(i->acl_access);
-        acl_free(i->acl_default);
+        if (i->acl_access)
+                acl_free(i->acl_access);
+
+        if (i->acl_default)
+                acl_free(i->acl_default);
 #endif
 }