Fix rendering of blocks with labels

If a variable is a 1-level factor, it fails to draw label rectangles. To reproduce the bug try: 

tit <- as.data.frame(Titanic)
tit <- subset(tit, Class == 'Crew')
tit2d <- aggregate( Freq ~ Class + Survived, data=tit, sum);
alluvial( tit2d[,1:2], freq=tit2d$Freq))