CDRIVER-5634 account for embedded NULL when copying salt (#1700)

* CDRIVER-5634 account for embedded NULL when copying `salt`

* remove unnecessary casts

Author: kevinAlbs

src/libmongoc/src/mongoc/mongoc-crypto-cng.c

@@ -176,13 +176,15 @@ _bcrypt_derive_key_pbkdf2 (BCRYPT_ALG_HANDLE prf,
    }
 
    // Make non-const versions of password and salt.
-   char *password_copy = bson_strndup (password, password_len);
-   char *salt_copy = bson_strndup ((const char *) salt, salt_len);
+   unsigned char *password_copy = bson_malloc (password_len);
+   memcpy (password_copy, password, password_len);
+   unsigned char *salt_copy = bson_malloc (salt_len);
+   memcpy (salt_copy, salt, salt_len);
 
    NTSTATUS status = BCryptDeriveKeyPBKDF2 (prf,
-                                            (unsigned char *) password_copy,
+                                            password_copy,
                                             (ULONG) password_len,
-                                            (unsigned char *) salt_copy,
+                                            salt_copy,
                                             (ULONG) salt_len,
                                             (ULONGLONG) iterations,
                                             output,