tls,http2: handle writes after SSL destroy more gracefully

This might otherwise result in a hard crash when trying
to write to a socket after a sudden disconnect.

Note that the test here uses an aborted `h2load` run to create
the failing requests; That’s far from ideal, but it provides
a reasonably reliably reproduction at this point.

src/tls_wrap.cc

@@ -553,7 +553,12 @@ int TLSWrap::DoWrite(WriteWrap* w,
                      size_t count,
                      uv_stream_t* send_handle) {
   CHECK_EQ(send_handle, nullptr);
-  CHECK_NE(ssl_, nullptr);
+
+  if (ssl_ == nullptr) {
+    ClearError();
+    error_ = "Write after DestroySSL";
+    return UV_EPROTO;
+  }
 
   bool empty = true;
 
@@ -593,12 +598,6 @@ int TLSWrap::DoWrite(WriteWrap* w,
     return 0;
   }
 
-  if (ssl_ == nullptr) {
-    ClearError();
-    error_ = "Write after DestroySSL";
-    return UV_EPROTO;
-  }
-
   crypto::MarkPopErrorOnReturn mark_pop_error_on_return;
 
   int written = 0;

test/parallel/test-http2-tls-disconnect.js

@@ -0,0 +1,32 @@
+'use strict';
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+
+const child_process = require('child_process');
+const http2 = require('http2');
+const fs = require('fs');
+
+const key = fixtures.readKey('agent8-key.pem', 'binary');
+const cert = fixtures.readKey('agent8-cert.pem', 'binary');
+
+const server = http2.createSecureServer({ key, cert }, (request, response) => {
+  fs.createReadStream(process.execPath).pipe(response);
+});
+
+// This should be doable with a reproduction purely written in Node;
+// that just requires somebody to take the time and actually do it.
+server.listen(0, () => {
+  const proc = child_process.spawn('h2load', [
+    '-n', '1000',
+    `https://localhost:${server.address().port}/`
+  ]);
+  proc.on('error', (err) => {
+    if (err.code === 'ENOENT')
+      common.skip('no h2load');
+  });
+  proc.on('exit', () => server.close());
+  setTimeout(() => proc.kill(2), 100);
+});