Make Host::serialize able to serialize empty address lists

Previously this would produce a malformed packet as far as glibc is
concerned.

If there are no addresses the "found" field should not be one and the
other lookup fields can also be left blank.

src/handlers.rs

@@ -407,23 +407,40 @@ impl Host {
         }
 
         let num_addrs = num_v4 + num_v6;
+        let has_addrs = num_addrs > 0;
 
         let hostname_c_string_bytes = CString::new(self.hostname.clone())?.into_bytes_with_nul();
-        let hostname_c_string_len = hostname_c_string_bytes.len();
+        let hostname_c_string_len = if has_addrs {
+            hostname_c_string_bytes.len() as i32
+        } else {
+            0
+        };
 
         let header = protocol::HstResponseHeader {
             version: protocol::VERSION,
-            found: 1 as i32,
-            h_name_len: hostname_c_string_len as i32,
-            h_aliases_cnt: 0 as i32,
-            h_addrtype: if num_v4 != 0 {
+            found: if has_addrs { 1 } else { 0 },
+            h_name_len: hostname_c_string_len,
+            h_aliases_cnt: 0,
+            h_addrtype: if !has_addrs {
+                -1
+            } else if num_v4 != 0 {
                 nix::sys::socket::AddressFamily::Inet as i32
             } else {
                 nix::sys::socket::AddressFamily::Inet6 as i32
             },
-            h_length: if num_v4 != 0 { 4 as i32 } else { 16 as i32 },
+            h_length: if !has_addrs {
+                -1
+            } else if num_v4 != 0 {
+                4
+            } else {
+                16
+            },
             h_addr_list_cnt: num_addrs as i32,
-            error: 0,
+            error: if has_addrs {
+                0
+            } else {
+                protocol::H_ERRNO_HOST_NOT_FOUND
+            },
         };
 
         let total_len = 4 * 8 + hostname_c_string_len as i32 + buf_addrs.len() as i32;
@@ -433,14 +450,16 @@ impl Host {
         buf.extend_from_slice(header.as_slice());
 
         // add hostname
-        buf.extend_from_slice(&hostname_c_string_bytes);
+        if has_addrs {
+            buf.extend_from_slice(&hostname_c_string_bytes);
 
-        // add serialized addresses from buf_addrs
-        buf.extend_from_slice(buf_addrs.as_slice());
+            // add serialized addresses from buf_addrs
+            buf.extend_from_slice(buf_addrs.as_slice());
+        }
 
         debug_assert_eq!(buf.len() as i32, total_len);
 
-	Ok(buf)
+        Ok(buf)
     }
 }