Ubuntu 18.04 crashes when multi-argument UDAF is called as a subquery.

Regression testing has shown that the following query crashes ExeMgr:
select f, count from (select CCHAR9, regr_avgx(CDECIMAL9_2, length(CCHAR9)) f from datatypetestm1 group by 1) a group by f order by f desc;

The crash happens at tuppleaggregatestep.cpp:3800 with j being 1 but the array it's indexing having only one element and thus index is out of bounds.