opcontrol with default event

[Will C. <wc...@re...>]

I found that the code in opcontrol does not handle using the default 
event. In the case were the default event is used. NR_CHOSEN is 0 and 
there no array CHOSEN_EVENTS. This caused a problem in 
normalise_events(). The for loop would have a `seq 0 -1`. The attached 
patch tests for that situation and skips the processing if there are no 
events.

Okay to checkin?

-Will