src: fix UB in InternalModuleReadFile()

`&vec[0]` is undefined behavior when `vec.size() == 0`.

It is mostly academic because package.json files are not usually empty
and because with most STL implementations it decays to something that
is legal C++ as long as the result is not dereferenced, but better safe
than sorry.

Note that the tests don't actually fail because of that, I added them
as sanity checks.

PR-URL: https://github.com/nodejs/node/pull/16871
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/test/parallel/test-module-binding.js b/test/parallel/test-module-binding.js
new file mode 100644
index 0000000000..ba11c3e47e
--- /dev/null
+++ b/test/parallel/test-module-binding.js
@@ -0,0 +1,9 @@
+'use strict';
+require('../common');
+const fixtures = require('../common/fixtures');
+const { internalModuleReadFile } = process.binding('fs');
+const { strictEqual } = require('assert');
+
+strictEqual(internalModuleReadFile('nosuchfile'), undefined);
+strictEqual(internalModuleReadFile(fixtures.path('empty.txt')), '');
+strictEqual(internalModuleReadFile(fixtures.path('empty-with-bom.txt')), '');