json_pointer: move array out-of-bounds check outside of is_valid_index()

The out-of-bounds check is useful when trying to index/obtain a value from
an array.
However, when we set a value to a specific JSON pointer, we can allow
values that are outside the length of the current array.
The RFC6901 doc isn't clear on that aspect, and doing so is a bit more
in-line with how json_object_array_{put,insert}_idx() functions behave.

This changes the behavior of json_pointer_set{f}() because now a value can
be set anywhere in the array.

Also, added a test-case for this behavior change.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

tests/test_json_pointer.c

@@ -269,6 +269,22 @@ static void test_example_set(void)
 	printf("%s\n", json_object_get_string(jo1));
 
 	json_object_put(jo1);
+
+	jo1 = json_tokener_parse("[0, 1, 2, 3]");
+	jo2 = json_tokener_parse("[0, 1, 2, 3, null, null, null, 7]");
+
+	assert(0 == json_pointer_set(&jo1, "/7", json_object_new_int(7)));
+	assert(1 == json_object_equal(jo1, jo2));
+
+	json_object_put(jo1);
+
+	jo1 = json_tokener_parse("[0, 1, 2, 3]");
+
+	assert(0 == json_pointer_setf(&jo1, json_object_new_int(7), "/%u", 7));
+	assert(1 == json_object_equal(jo1, jo2));
+
+	json_object_put(jo1);
+	json_object_put(jo2);
 }
 
 static void test_wrong_inputs_set(void)