json_pointer: find null array elements

json_pointer_get_single_path() returned ENOENT when the array element
at an in-range index was NULL, which is how json-c stores a JSON null.
A null object member was already found, but on [0, null, 2] "/1" failed
with "not found", although RFC 6901 says it references the null value.

The array index is bounds-checked just before, so a NULL element now
resolves like a null member: json_pointer_get() returns 0 and sets
*res to NULL. This also fixes json_patch_apply(), which failed every
"test", "replace" and "remove" on a null array element, and every
"move" and "copy" from one, with "Did not find element referenced by
path field".

Added a json_pointer_get() test, and json_patch_tests.json cases that
mirror the existing null object member ones.

tests/test_json_pointer.c

@@ -122,6 +122,18 @@ static void test_example_get(void)
 		test_example_int(jo1, json_pointers[i].s, json_pointers[i].i);
 
 	json_object_put(jo1);
+
+	/* Test /1 == null, a null array element is found and returned as NULL */
+	jo1 = json_tokener_parse("[0, null, 2]");
+	jo2 = jo1;
+	assert(0 == json_pointer_get(jo1, "/1", NULL));
+	assert(0 == json_pointer_get(jo1, "/1", &jo2));
+	assert(NULL == jo2);
+	errno = 0;
+	assert(0 != json_pointer_get(jo1, "/1/foo", NULL));
+	assert(errno == ENOENT);
+	printf("PASSED - GET - /1 == null\n");
+	json_object_put(jo1);
 }
 
 /* I'm not too happy with the RFC example to test the recursion of the json_pointer_get() function */