src,inspector: fix empty MaybeLocal crash

Return early when the Inspector StringView to V8 String conversion fails
and returns an empty MaybeLocal instead of running the invalid
ToLocalChecked() assertion.

Fixes: #42407
Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #42409
Backport-PR-URL: #42967
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/inspector_js_api.cc

@@ -75,10 +75,10 @@ class JSBindingsConnection : public AsyncWrap {
       Isolate* isolate = env_->isolate();
       HandleScope handle_scope(isolate);
       Context::Scope context_scope(env_->context());
-      MaybeLocal<String> v8string =
-          String::NewFromTwoByte(isolate, message.characters16(),
-                                 NewStringType::kNormal, message.length());
-      Local<Value> argument = v8string.ToLocalChecked().As<Value>();
+      Local<Value> argument;
+      if (!String::NewFromTwoByte(isolate, message.characters16(),
+                                  NewStringType::kNormal,
+                                  message.length()).ToLocal(&argument)) return;
       connection_->OnMessage(argument);
     }
 

test/pummel/test-repl-empty-maybelocal-crash.js

@@ -0,0 +1,20 @@
+'use strict';
+const common = require('../common');
+
+if (process.config.variables.arm_version === '7') {
+  common.skip('Too slow for armv7 bots');
+}
+
+// The process should not crash when the REPL receives the string, 'ss'.
+// Test for https://github.com/nodejs/node/issues/42407.
+
+const repl = require('repl');
+
+const r = repl.start();
+
+r.write('var buf = Buffer.from({length:200e6},(_,i) => i%256);\n');
+r.write('var ss = buf.toString("binary");\n');
+r.write('ss');
+r.write('.');
+
+r.close();