net/arp: Fix uninitialized scalar variable

net/arp: Fix uninitialized scalar variable

In the function grub_net_arp_receive(), grub_net_network_level_address_t
sender_addr and target_addr are being called but aren't being initialized.
In both of these structs, each member is being set to a value except for
grub_dns_option_t option. This results in this member being filled with junk
data from the stack. To prevent this, we can set the option member in both
structs to 0.

Fixes: CID 375030
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>