http: fix error return in Finish()

`http_parser_execute(..., nullptr, 0)` returns either `0` or `1`. The
expectation is that no error must be returned if it is `0`, and if
it is `1` - a `Error` object must be returned back to user.

The introduction of `llhttp` and the refactor that happened during it
accidentally removed the error-returning code. This commit reverts it
back to its original state.

Fix: #24585
PR-URL: https://github.com/nodejs/node/pull/24738
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-parser-finish-error.js

@@ -0,0 +1,27 @@
+'use strict';
+
+const common = require('../common');
+const net = require('net');
+const http = require('http');
+const assert = require('assert');
+
+const str = 'GET / HTTP/1.1\r\n' +
+            'Content-Length:';
+
+
+const server = http.createServer(common.mustNotCall());
+server.on('clientError', common.mustCall((err, socket) => {
+  assert(/^Parse Error/.test(err.message));
+  assert.strictEqual(err.code, 'HPE_INVALID_EOF_STATE');
+  socket.destroy();
+}, 1));
+server.listen(0, () => {
+  const client = net.connect({ port: server.address().port }, () => {
+    client.on('data', common.mustNotCall());
+    client.on('end', common.mustCall(() => {
+      server.close();
+    }));
+    client.write(str);
+    client.end();
+  });
+});