sd-dhcp-server: propagate error on parsing DHCP packet

Otherwise, we silently ignore the received packet and that makes hard to
debug issue.

(cherry picked from commit 809da72)
(cherry picked from commit 7483f75)

Author: yuwata

src/libsystemd-network/sd-dhcp-server.c

@@ -1079,7 +1079,7 @@ int dhcp_server_handle_message(sd_dhcp_server *server, DHCPMessage *message, siz
 
         type = dhcp_option_parse(message, length, parse_request, req, &error_message);
         if (type < 0)
-                return 0;
+                return type;
 
         r = ensure_sane_request(server, req, message);
         if (r < 0)

src/libsystemd-network/test-dhcp-server.c

@@ -138,7 +138,7 @@ static void test_message_handler(void) {
         test.option_type.code = 0;
         test.option_type.length = 0;
         test.option_type.type = 0;
-        assert_se(dhcp_server_handle_message(server, (DHCPMessage*)&test, sizeof(test)) == 0);
+        assert_se(dhcp_server_handle_message(server, (DHCPMessage*)&test, sizeof(test)) == -ENOMSG);
         test.option_type.code = SD_DHCP_OPTION_MESSAGE_TYPE;
         test.option_type.length = 1;
         test.option_type.type = DHCP_DISCOVER;