Use cbor_value_get_next_byte instead of accessing structure directly

Signed-off-by: linraymond2006 <linraymond2006@gmail.com>

examples/simplereader.c

@@ -178,7 +178,7 @@ int main(int argc, char **argv)
 
     if (err) {
         fprintf(stderr, "CBOR parsing failure at offset %ld: %s\n",
-                it.source.ptr - buf, cbor_error_string(err));
+                cbor_value_get_next_byte(&it) - buf, cbor_error_string(err));
         return 1;
     }
     return 0;