ARP/NDP does not send solicitation when no source address is available

ARP/NDP does not send solicitation when no source address is available

Currently, when the interface has no address assigned to it,
VPP crashes as ip_interface_address_for_packet assumes there
always is at least one address.

This patch checks if an address is available at all.
The only part of VPP using it is ARP or NDP.
When a neighbor discovery message has to be sent while
there is no address assigned on the interace, no
solicitation is sent and an error counter is incremented.

Change-Id: Ia9fdaf8e84050a1ceeb47f5370819d3df95714f0
Signed-off-by: Pierre Pfister <ppfister@cisco.com>