fixing division by 0.

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15999 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
fixing division by 0. (previous check-in was premature)

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@16000 29f149b2-0c63-11e1-8f5c-a347f6a9acd0