No panic if no valid src addr for DNS query

Don't panic if there is no valid source address for the DNS query. The
state of the query is set to `Failed` instead and the query is
abandoned.

src/socket/dns.rs

@@ -612,7 +612,15 @@ impl<'a> Socket<'a> {
                 };
 
                 let dst_addr = servers[pq.server_idx];
-                let src_addr = cx.get_source_address(&dst_addr).unwrap(); // TODO remove unwrap
+                let src_addr = match cx.get_source_address(&dst_addr) {
+                    Some(src_addr) => src_addr,
+                    None => {
+                        net_trace!("no source address for destination {}", dst_addr);
+                        q.set_state(State::Failure);
+                        continue;
+                    }
+                };
+
                 let ip_repr = IpRepr::new(
                     src_addr,
                     dst_addr,