Avoid problems with object's serial number growing too big

Transform serial key comparison from substraction to real comparison.
Reset serial to zero in json_object_clear() to avoid it growing out of
bounds when reusing objects.

Closes GH-40.
Closes GH-41.

src/dump.c

@@ -159,8 +159,10 @@ static int object_key_compare_keys(const void *key1, const void *key2)
 
 static int object_key_compare_serials(const void *key1, const void *key2)
 {
-    return (*(const object_key_t **)key1)->serial -
-           (*(const object_key_t **)key2)->serial;
+    size_t a = (*(const object_key_t **)key1)->serial;
+    size_t b = (*(const object_key_t **)key2)->serial;
+
+    return a < b ? -1 : a == b ? 0 : 1;
 }
 
 static int do_dump(const json_t *json, size_t flags, int depth,

src/value.c

@@ -186,7 +186,9 @@ int json_object_clear(json_t *json)
         return -1;
 
     object = json_to_object(json);
+
     hashtable_clear(&object->hashtable);
+    object->serial = 0;
 
     return 0;
 }