Refine memory management (#579)

- Added one macro to avoid dangling pointers
- Added two functions to perform NULL pointer check
  since the allocation is not guaranteed by C library, although it
  is a rare case, just for sanity
- Add NULL pointer check to brealloc() and bfree() and for sanity as well

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

src/json.c

@@ -28,6 +28,7 @@
  */
 
 #include "json.h"
+#include "mm-wrapper.h"
 
 #ifdef _MSC_VER
 #ifndef _CRT_SECURE_NO_WARNINGS
@@ -90,12 +91,12 @@ typedef struct {
 
 static void *default_alloc(size_t size, int zero, void *user_data)
 {
-    return zero ? calloc(1, size) : malloc(size);
+    return zero ? calloc(1, size) : SS_SAFEMALLOC(size);
 }
 
 static void default_free(void *ptr, void *user_data)
 {
-    free(ptr);
+    SS_SAFEFREE(ptr);
 }
 
 static void *json_alloc(json_state *state, unsigned long size, int zero)