Cindy: Fixing trigger for next wave

A next wave was triggered when only one Yeti was alive. This means when
during the 10th wave only one last Yeti was alive, then every 5 seconds
$@FIGHT_YETI_WAVE was incremented. If the last Yeti was not killed within
one minute, then wave 22 was reached and the deadly Yeti spawn happend.

This bug fix also resolves the timing problem mentioned in the comment.