Fix c++20 compilation problem for clang10

Author: Christian Ledergerber

include/json/allocator.h

@@ -35,11 +35,12 @@ template <typename T> class SecureAllocator {
    * Release memory which was allocated for N items at pointer P.
    *
    * The memory block is filled with zeroes before being released.
-   * The pointer argument is tagged as "volatile" to prevent the
-   * compiler optimizing out this critical step.
+   * The pointer argument passed to memset is tagged as "volatile" to prevent
+   * the compiler optimizing out this critical step.
    */
-  void deallocate(volatile pointer p, size_type n) {
-    std::memset(p, 0, n * sizeof(T));
+  void deallocate(pointer p, size_type n) {
+    volatile pointer do_not_optimize_memset_p = p;
+    std::memset(do_not_optimize_memset_p, 0, n * sizeof(T));
     // free using "global operator delete"
     ::operator delete(p);
   }