Merge: contrib/opportunity: prevent null receiver error

This error crashed the server 52 times in the last month.

Opportunity on xymus.net has already been updated.

Thanks to @isra17 and @ageei exec members for forging broken requests and revealing this problem.

Pull-Request: #1209
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc contrib/opportunity/src/opportunity_controller.nit
Simple merge