Fix unsafe cast and detect resize overflow. (#31106)

Summary:
Removing unsafe cast from `int` to `uint16_t`.
Also, adding code to detect multiplication overflow during buffer resize.

## Changelog

<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->

[General] [Fix] - Fix unsafe cast and detect overflow in MapBuffer.

Pull Request resolved: https://github.com/facebook/react-native/pull/31106

Test Plan: Code compiles in Visual Studio 2019 without the unsafe cast warning (or error depending on the configuration).

Reviewed By: mdvacca

Differential Revision: D26865138

Pulled By: rozele

fbshipit-source-id: 4692a38b05fc873e31fbbe94d70803244e82de5d

ReactCommon/react/renderer/mapbuffer/MapBuffer.cpp

@@ -12,7 +12,7 @@ using namespace facebook::react;
 namespace facebook {
 namespace react {
 
-MapBuffer::MapBuffer(int initialSize) {
+MapBuffer::MapBuffer(uint16_t initialSize) {
   _dataSize = initialSize;
   _data = new Byte[_dataSize];
   // TODO: Should we clean up memory here?
@@ -20,6 +20,13 @@ MapBuffer::MapBuffer(int initialSize) {
 
 void MapBuffer::makeSpace() {
   int oldDataSize = _dataSize;
+  if (_dataSize >= std::numeric_limits<uint16_t>::max() / 2) {
+    LOG(ERROR)
+        << "Error: trying to assign a value beyond the capacity of uint16_t"
+        << static_cast<uint32_t>(_dataSize) * 2;
+    throw "Error: trying to assign a value beyond the capacity of uint16_t" +
+        std::to_string(static_cast<uint32_t>(_dataSize) * 2);
+  }
   _dataSize *= 2;
   uint8_t *_newdata = new Byte[_dataSize];
   uint8_t *_oldData = _data;

ReactCommon/react/renderer/mapbuffer/MapBuffer.h

@@ -13,7 +13,7 @@ namespace facebook {
 namespace react {
 
 // 506 = 5 entries = 50*10 + 6 sizeof(header)
-const int INITIAL_SIZE = 506;
+constexpr uint16_t INITIAL_SIZE = 506;
 
 /**
  * MapBuffer is an optimized map format for transferring data like props between
@@ -46,7 +46,7 @@ class MapBuffer {
  public:
   MapBuffer() : MapBuffer(INITIAL_SIZE) {}
 
-  MapBuffer(int initialSize);
+  MapBuffer(uint16_t initialSize);
 
   ~MapBuffer();