nat: VPP crashes when route is not reachable

The problem was reproducible only with icmp packet type
when det44 in, out interfaces were swapped.
Dst addr was unknown but packet has been forwarded.

Type: fix
Ticket: VPP-1958

Signed-off-by: Daniel Béreš <daniel.beres@pantheon.tech>
Change-Id: Ie446cf2ac866955cc668fe2848f954a2ef92e3fa

src/plugins/nat/det44/det44_out2in.c

@@ -173,6 +173,9 @@ icmp_match_out2in_det (vlib_node_runtime_t * node,
 	}
       det44_log_info ("unknown dst address:  %U",
 		      format_ip4_address, &ip0->dst_address);
+      b0->error = node->errors[DET44_OUT2IN_ERROR_NO_TRANSLATION];
+      next0 = DET44_OUT2IN_NEXT_DROP;
+
       goto out;
     }