Fix assert when getting empty string at end of storage

Summary:
When the empty string is the last entry in the string table, our code would illegally use std::vector::operator[] past the last char, causing assert failures in debug builds.

Fix this by using pointer arithmetic instead.

Reviewed By: jpporto

Differential Revision: D36551789

fbshipit-source-id: 41380716a2e4a5148236518b560183ce6c5d972e

lib/BCGen/HBC/ConsecutiveStringStorage.cpp

@@ -758,8 +758,12 @@ uint32_t ConsecutiveStringStorage::getEntryHash(size_t i) const {
   ensureStorageValid();
 
   auto &entry = strTable_[i];
-  const unsigned char *data = &storage_[entry.getOffset()];
   uint32_t length = entry.getLength();
+  assert(
+      entry.getOffset() + (entry.isUTF16() ? length * 2 : length) <=
+          storage_.size() &&
+      "entry past end");
+  const unsigned char *data = storage_.data() + entry.getOffset();
   if (entry.isUTF16()) {
     const char16_t *u16data = reinterpret_cast<const char16_t *>(data);
     return hermes::hashString(ArrayRef<char16_t>{u16data, length});

test/hermes/empty-string-lazy.js

@@ -0,0 +1,11 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// RUN: %hermes -O -target=HBC %s
+// RUN: %hermes -O -target=HBC -lazy %s
+// RUN: %hermes -O -target=HBC -emit-binary -out %t.hbc %s && %hermes %t.hbc
+''['']