Check for zero duration before skipping step. This fixes pattern

    echo "0 1000 10 2550 0 1000" > pattern

which should do [  .-xXx-.] but does [   Xx-.]
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Suggested-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>