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)

Author: yuwata

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

@@ -1121,7 +1121,7 @@ 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

@@ -146,7 +146,7 @@ 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;