xtables-restore: fix for --noflush and empty lines

xtables-restore: fix for --noflush and empty lines

Lookahead buffer used for cache requirements estimate in restore
--noflush separates individual lines with nul-chars. Two consecutive
nul-chars are interpreted as end of buffer and remaining buffer content
is skipped.

Sadly, reading an empty line (i.e., one containing a newline character
only) caused double nul-chars to appear in buffer as well, leading to
premature stop when reading cached lines from buffer.

To fix that, make use of xtables_restore_parse_line() skipping empty
lines without calling strtok() and just leave the newline character in
place. A more intuitive approach, namely skipping empty lines while
buffering, is deliberately not chosen as that would cause wrong values
in 'line' variable.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1400
Fixes: 09cb517949e69 ("xtables-restore: Improve performance of --noflush operation")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Arturo Borrero Gonzalez <arturo@netfilter.org>