fix bug in get-text-extent on zero-length string when neither descent or vertical space is requested
fix bug in get-text-extent on zero-length string when neither descent or vertical space is requested

svn: r6776

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket