#145 destroy trial crash if last of more than one

[Michele Gubian]

quietly include Models/ia.eNm
quietly ia stage

in case ia.eNm contains only one trial, say brief_masked
destroy brief_masked
OK

while if ia.eNm contains more than one trial (brief_mask and present), destroying the last one crashes lazynut.
Note: it does not matter if you use ia add_script or include to include the trial(s).