net: icmp: Fix Echo Replies with unspecified address

Fix two issues with sending ICMP Echo Reply for requests sent for
multicast address:
* Use the originator IP address instead of multicast when selecting
  source address for the reply
* In case no address match is found, drop the packet, instead of
  replying with unspecified address.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>