Before 741b494f, when the reference list
modification description was invalid, the code would substitute the
corresponding reference from the initial ("default") reference list.
After that commit, it will just return an error.

Since there are apparently invalid samples in the wild that used to play
fine with the old code, it is a good idea to re-add some sort of error
resilience here. So, when the reference list modification results in a
missing frame, substitute a previous reference frame for it. The
relevant sample again decodes fine with the same output as previously.