Fix a crash when viewing statistics including a score on a course which is not found.

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#	modified:   scorecard.pro
#	modified:   src/main-window.cpp
#	modified:   src/score-dialog.cpp
#	modified:   src/stat-model.cpp
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#	Makefile
#	bin/
#	build-stamp
#	builddir/
#	debian/files
#	debian/scorecard.substvars
#	debian/scorecard/
#	debug/
#	images/26/
#	images/48/
#	images/64/
#	moc/
#	obj/
#	qrc_scorecard.cpp
#	queries/
#	scorecard.pro.user
#	scorecard.pro.user.1.3
