Description:
------------
Right now, if json_encode sees wrong UTF-8 data, it just cuts the string in the middle, no error returned, no message produced. 

I think it's not a good idea to just silently cut the data. In fact, I think it is a bug caused by this code in ext/json/utf8_to_utf16.c:
        if (c < 0) {
            return UTF8_END ? the_index : UTF8_ERROR;
        }
which inherited this bug from code published on json.org. It should be:
        if (c < 0) {
            return (c == UTF8_END) ? the_index : UTF8_ERROR;
        }

Reproduce code:
---------------
var_dump(json_encode("ab\xE0"));
var_dump(json_encode("ab\xE0\""));

Expected result:
----------------
Some error message

Actual result:
--------------
Just:
""ab""
""ab""