net/net: Fix uninitialized scalar variable

net/net: Fix uninitialized scalar variable

In the function grub_net_ipv6_get_link_local(), grub_net_network_level_address_t
addr is called but isn't being initialized. This results in the member
grub_dns_option_t option being filled with junk data from the stack. We can
prevent this by setting the option member in addr to 0.

Fixes: CID 375033
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>