xtables-restore: Fix parser feed from line buffer

xtables-restore: Fix parser feed from line buffer

When called with --noflush, xtables-restore would trip over chain lines:
Parser uses strtok() to separate chain name, policy and counters which
inserts nul-chars into the source string. Therefore strlen() can't be
used anymore to find end of line. Fix this by caching line length before
calling xtables_restore_parse_line().

Fixes: 09cb517949e69 ("xtables-restore: Improve performance of --noflush operation")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>