Prior to this prohibition, such a trigger caused a crash.

Thomas Munro, per a report from Neha Sharma.  I added a
regression test.

Discussion: http://postgr.es/m/CAEepm=0VR5W-N38eTkO_FqJbGqQ_ykbBRmzmvHyxDhy1p=0Csw@mail.gmail.com