Fix overly-enthusiastic Assert in printing of Param reference expressions.

A NestLoopParam's value can only be a Var or Aggref, but this isn't the
case in general for SubPlan parameters, so print_parameter_expr had better
be prepared to cope.  Brain fade in my recent patch to print the referenced
expression instead of just printing $N for PARAM_EXEC Params.  Per report
from Pavel Stehule.