Opening the same score twice can cause crash

Mac OS X 10.7.4 b773d4831d

1. Open a score and change it a bit
2. Double click on the score again in the finder

MuseScore crashes with the following trace


0 net.sourceforge.mscore 0x0000000100012afc QBasicAtomicInt::ref() + 12
1 net.sourceforge.mscore 0x0000000100012f4a QString::QString(QString const&) + 90
2 net.sourceforge.mscore 0x00000001000123fd QString::QString(QString const&) + 29
3 net.sourceforge.mscore 0x00000001001c6053 Score::tmpName() const + 35
4 net.sourceforge.mscore 0x00000001001af144 MuseScore::removeTab(int) + 548
5 net.sourceforge.mscore 0x00000001001ad02b MuseScore::appendScore(Score*) + 203
6 net.sourceforge.mscore 0x00000001001bb47b MuseScore::handleMessage(QString const&) + 219
7 net.sourceforge.mscore 0x00000001001bb323 MuseScore::eventFilter(QObject*, QEvent*) + 83