owipcalc: fix contains not respect default route

owipcalc: fix contains not respect default route

In IPv4 the default route can be written as
 0.0.0.0/0

In IPv6 the default route can be written as
 ::/0

If u try
  owipcalc 0.0.0.0/0 contains 1.1.1.1
or
  owipcalc ::/0 contains ::1
owipcalc will respond with 0 meaning that the "default prefixes" do not
contain the routes.
That is why we check now for 0 prefix.

Furthermore, if the prefix is 0, i will be 16. We will access a negative
array entry in the line:
 uint8_t net1 = x->s6_addr[15-i] & m;

Divide by % 16 to prevent i becoming 16:
 uint8_t i = ((128 - a->prefix) / 8) % 16;

Signed-off-by: Nick Hainke <vincent@systemli.org>