http/server/Send: revert response.length calculation ordering

Was accidently swapped by commit c45aae2

Author: MaxKellermann

debian/changelog

@@ -4,6 +4,7 @@ cm4all-beng-proxy (21.18) unstable; urgency=low
   * bp: reduce MOUNT_LISTEN_STREAM idle timeout to 2 minutes
   * http_client: fix use-after-free bug
   * http_server: fix use-after-free bug
+  * http_server: do not log LENGTH attribute for response without body
   * istream/dechunk: fix integer overflow
 
  --   

src/http/server/Send.cxx

@@ -211,10 +211,6 @@ HttpServerConnection::SubmitResponse(HttpStatus status,
 	GrowingBuffer headers3 = headers.ToBuffer();
 	headers3.Write("\r\n"sv);
 
-	/* make sure the access logger gets a negative value if there
-	   is no response body */
-	response.length -= !body;
-
 #ifdef HAVE_URING
 	if (auto *uring_queue = socket->GetUringQueue()) {
 		assert(uring_send == nullptr);
@@ -232,6 +228,10 @@ HttpServerConnection::SubmitResponse(HttpStatus status,
 	assert(header_length.exhaustive);
 	response.length = -header_length.length;
 
+	/* make sure the access logger gets a negative value if there
+	   is no response body */
+	response.length -= !body;
+
 	SetResponseIstream(NewConcatIstream(request_pool, std::move(header_stream),
 					    std::move(body)));
 	DeferWrite();