fix(h2): do not rewind kPendingIdx past in-flight requests

lib/dispatcher/client-h2.js

@@ -744,7 +744,7 @@ function writeH2 (client, request) {
     requestFinalized = true
     client[kQueue][client[kRunningIdx]++] = null
 
-    if (resetPendingIdx) {
+    if (resetPendingIdx && client[kPendingIdx] < client[kRunningIdx]) {
       client[kPendingIdx] = client[kRunningIdx]
     }
 

test/http2-destroy-after-failed-stream.js

@@ -0,0 +1,40 @@
+'use strict'
+
+const { test } = require('node:test')
+const assert = require('node:assert')
+const { createServer } = require('node:http2')
+const { once } = require('node:events')
+const { setTimeout: sleep } = require('node:timers/promises')
+const { Agent } = require('..')
+
+test('closing dispatcher after one multiplexed stream failed pre-response does not crash', async (t) => {
+  const server = createServer()
+  server.on('stream', async (stream, headers) => {
+    if (headers[':path'] === '/slow') {
+      await sleep(50)
+      stream.respond({ ':status': 200 })
+      stream.end('slow')
+    } else {
+      stream.close()
+    }
+  })
+  server.listen(0)
+  await once(server, 'listening')
+  t.after(() => server.close())
+
+  const origin = `http://localhost:${server.address().port}`
+  const dispatcher = new Agent({ connections: 1, useH2c: true })
+
+  const slow = dispatcher
+    .request({ origin, path: '/slow', method: 'GET' })
+    .then((res) => res.body.text())
+
+  await assert.rejects(
+    dispatcher.request({ origin, path: '/bad', method: 'GET' }),
+    { name: 'InformationalError' }
+  )
+
+  assert.strictEqual(await slow, 'slow')
+
+  await dispatcher.close()
+})