MB-62221: Fix buffer overflow (#29)

- The `faiss::VectorIOWriter.data` is a vector of type `uint8_t`. However,
   the code attempts to copy this vector to a malloc'd array of type `unsigned char`.
   This results in platform-dependent code since `uint8_t` is platform-agnostic,
   whereas `unsigned char` is not defined consistently across different platforms
   in the C standard.
- This issue becomes apparent when using `std::copy`, where the effective
   signature is `std::copy(uint8_t*, uint8_t*, unsigned char*)`.
- According to the C++ STL template for `copy`, both the StartPointer and
   DestinationPointer are incremented by 1. The behavior of this pointer arithmetic
   depends on the type of pointer passed. Since `uint8_t*` is platform-agnostic
   and `unsigned char*` is platform-dependent, a buffer overflow could occur,
   leading to undefined behavior for `std::copy()` as per the C++ standard.
- Also fixes a mismatched header file where the signature of
   `faiss_read_index_buf` was not matching the one defined in the source cpp file.

c_api/index_io_c_ex.cpp

@@ -17,11 +17,11 @@
 using faiss::Index;
 using faiss::IndexBinary;
 
-int faiss_write_index_buf(const FaissIndex* idx, size_t* size, unsigned char** buf) {
+int faiss_write_index_buf(const FaissIndex* idx, size_t* size, uint8_t** buf) {
     try {
         faiss::VectorIOWriter writer;
         faiss::write_index(reinterpret_cast<const Index*>(idx), &writer);
-        unsigned char* tempBuf = (unsigned char*)malloc((writer.data.size()) * sizeof(uint8_t));
+        uint8_t* tempBuf = (uint8_t*)malloc((writer.data.size()) * sizeof(uint8_t));
         std::copy(writer.data.begin(), writer.data.end(), tempBuf);
         *buf = tempBuf;
         *size = writer.data.size();

c_api/index_io_c_ex.h

@@ -31,11 +31,11 @@ extern "C" {
 
 /** Write index to buffer
  */
-int faiss_write_index_buf(const FaissIndex* idx, size_t* buf_size, unsigned char** buf);
+int faiss_write_index_buf(const FaissIndex* idx, size_t* buf_size, uint8_t** buf);
 
 /** Read index from buffer
  */
-int faiss_read_index_buf(const unsigned char* buf, size_t limit, int io_flags,
+int faiss_read_index_buf(const uint8_t* buf, size_t limit, int io_flags,
         FaissIndex** p_out);
 
 #ifdef __cplusplus