crash in startup when trying to open recent score, which got removed meanwhile

1. set MuseScore preferences to start with last session
2. have some score open
3. exit MuseScore
4. remove (or rename) that score
5. start MuseScore

result: an alert box complaining about the missing score (expected), then a crash (not expected ;-))

cause: "addRecentScore(score);" in file.cpp gets called after "delete score; score = 0;", i.e. with a NULL pointer