Even if they are not valid they still "eat" all the 4/5/6 bytes, so they should be fixed too. I haven't see anything about these bytes in chapter 3 so far, but there are at least two possibilities:
1) consider all the bytes in range F5-FD as invalid without looking for the other bytes;
2) try to read the next 4/5/6 bytes and fail if they are no continuation bytes.
We can also look at what others do (e.g. browsers and other languages).