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.)

Objects/unicodeobject.c

@@ -634,7 +634,7 @@ int utf8_decoding_error(const char **source,
 #define UTF8_ERROR(details)  do {                       \
     if (utf8_decoding_error(&s, &p, errors, details))   \
         goto onError;                                   \
-    continue;                                           \
+    goto nextChar;                                      \
 } while (0)
 
 PyObject *PyUnicode_DecodeUTF8(const char *s,
@@ -731,6 +731,7 @@ PyObject *PyUnicode_DecodeUTF8(const char *s,
             UTF8_ERROR("unsupported Unicode code range");
         }
         s += n;
+nextChar:
     }
 
     /* Adjust length */