net: arp: Exit early on zero gateway provided

In a case gateway is not set drop packet early otherwise bogus
ARPs for 0.0.0.0 are sent.

...
[net/arp] [DBG] find_entry: (0xa8006720): dst 0.0.0.0
[net/arp] [DBG] find_entry: (0xa8006720): [0] iface 0xa800cd80 dst
0.0.0.0 ll 00:00:00:00:00:00 pending 0xa800a7c0
[net/arp] [DBG] find_entry: (0xa8006720): ARP already pending to
0.0.0.0 ll 00:00:00:00:00:00
[net/arp] [DBG] net_arp_prepare: (0xa8006720): Resending ARP
0xa800a380
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>