Fix deallocate for working on old compiers (#1478)

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

include/json/allocator.h

@@ -6,6 +6,7 @@
 #ifndef JSON_ALLOCATOR_H_INCLUDED
 #define JSON_ALLOCATOR_H_INCLUDED
 
+#include <algorithm>
 #include <cstring>
 #include <memory>
 
@@ -38,8 +39,16 @@ public:
    * The memory block is filled with zeroes before being released.
    */
   void deallocate(pointer p, size_type n) {
-    // memset_s is used because memset may be optimized away by the compiler
+    // These constructs will not be removed by the compiler during optimization,
+    // unlike memset.
+#if defined(HAVE_MEMSET_S)
     memset_s(p, n * sizeof(T), 0, n * sizeof(T));
+#elif defined(_WIN32)
+    RtlSecureZeroMemory(p, n * sizeof(T));
+#else
+    std::fill_n(reinterpret_cast<volatile unsigned char*>(p), n, 0);
+#endif
+
     // free using "global operator delete"
     ::operator delete(p);
   }