Fix to a bug found by Florian Weimer:

Fix to a bug found by Florian Weimer:

The UTF-8 decoder is still buggy (i.e. it doesn't pass Markus Kuhn's
stress test), mainly due to the following construct:

    #define UTF8_ERROR(details)  do {                       \
        if (utf8_decoding_error(&s, &p, errors, details))   \
            goto onError;                                   \
        continue;                                           \
    } while (0)

(The "continue" statement is supposed to exit from the outer loop,
but of course, it doesn't.  Indeed, this is a marvelous example of
the dangers of the C programming language and especially of the C
preprocessor.)