libxt_*limit: avoid division by zero

libxt_*limit: avoid division by zero

It was possible to specify -A mychain -m hashlimit --hashlimit
600059/minute; this would convert to r->avg=0, which subsequently
causes a division by zero when printing with -S mychain.

1. Avoid division by zero in print_rate by printing infinity
   instead.
2. Rewrite the test in parse_rate to properly reject too high rates.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>