test: read proper inspector message size

Fix a bug when messages bigger than 64kb where incorrectly parsed by
the inspector-helper.

PR-URL: #14596
Fixes: #14507
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/inspector/inspector-helper.js

@@ -53,6 +53,7 @@ function sendEnd(socket) {
 }
 
 function parseWSFrame(buffer, handler) {
+  // Protocol described in https://tools.ietf.org/html/rfc6455#section-5
   if (buffer.length < 2)
     return 0;
   if (buffer[0] === 0x88 && buffer[1] === 0x00) {
@@ -68,7 +69,8 @@ function parseWSFrame(buffer, handler) {
     dataLen = buffer.readUInt16BE(2);
     bodyOffset = 4;
   } else if (dataLen === 127) {
-    dataLen = buffer.readUInt32BE(2);
+    assert(buffer[2] === 0 && buffer[3] === 0, 'Inspector message too big');
+    dataLen = buffer.readUIntBE(4, 6);
     bodyOffset = 10;
   }
   if (buffer.length < bodyOffset + dataLen)