afterUpdate call: fixed undefined opt

Hello, 

One of the two hooks introduced in the last release of Mapael is causing a type error when triggering an update. Actually when trying to execute `afterUpdate` function. The variable `opt` is undefined.
You can see it here: http://jsfiddle.net/neveldo/qGwWr/embedded/result/
I just added a line to check if the variable is defined before trigering `afterUpdate`.