luci-lib-ip: explicitly initialize address scope value

Explicitly initialize the address scope value to zero in parse_cidr() and
L_setaddr() to avoid stray scope identifiers getting added when formatting
the address object as string.

Fixes: 1d2b4c777 ("luci-lib-ip: support scoped IPv6 addresses")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>