src: fix setting Converter sub char length

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: #38330

PR-URL: #38331
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

src/node_i18n.cc

@@ -147,8 +147,13 @@ MaybeLocal<Object> Transcode(Environment* env,
   *status = U_ZERO_ERROR;
   MaybeLocal<Object> ret;
   MaybeStackBuffer<char> result;
-  Converter to(toEncoding, "?");
+  Converter to(toEncoding);
   Converter from(fromEncoding);
+
+  size_t sublen = ucnv_getMinCharSize(to.conv());
+  std::string sub(sublen, '?');
+  to.set_subst_chars(sub.c_str());
+
   const uint32_t limit = source_length * to.max_char_size();
   result.AllocateSufficientStorage(limit);
   char* target = *result;
@@ -189,7 +194,12 @@ MaybeLocal<Object> TranscodeFromUcs2(Environment* env,
   *status = U_ZERO_ERROR;
   MaybeStackBuffer<UChar> sourcebuf;
   MaybeLocal<Object> ret;
-  Converter to(toEncoding, "?");
+  Converter to(toEncoding);
+
+  size_t sublen = ucnv_getMinCharSize(to.conv());
+  std::string sub(sublen, '?');
+  to.set_subst_chars(sub.c_str());
+
   const size_t length_in_chars = source_length / sizeof(UChar);
   CopySourceBuffer(&sourcebuf, source, source_length, length_in_chars);
   MaybeStackBuffer<char> destbuf(length_in_chars);

test/parallel/test-icu-transcode.js

@@ -83,3 +83,8 @@ assert.deepStrictEqual(
   const dest = buffer.transcode(new Uint8Array(), 'utf8', 'latin1');
   assert.strictEqual(dest.length, 0);
 }
+
+// Test that it doesn't crash
+{
+  buffer.transcode(new buffer.SlowBuffer(1), 'utf16le', 'ucs2');
+}