doc: Set all memory to zero in secure_free

doc/apiref.rst

@@ -1476,7 +1476,7 @@ JSON structures by zeroing all memory when freed::
         ptr -= 8;
         size = *((size_t *)ptr);
 
-        guaranteed_memset(ptr, 0, size);
+        guaranteed_memset(ptr, 0, size + 8);
         free(ptr);
     }
 

test/suites/api/test_memory_funcs.c

@@ -65,7 +65,7 @@ static void secure_free(void *ptr)
     ptr = (char *)ptr - 8;
     size = *((size_t *)ptr);
 
-    /*guaranteed_*/memset(ptr, 0, size);
+    /*guaranteed_*/memset(ptr, 0, size + 8);
     free(ptr);
 }