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

techdragon/Postgres-XL,ashwinstar/gpdb,tpostgres-projects/tPostgres,oberstet/postgres-xl,jmcatamney/gpdb,zeroae/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,tpostgres-projects/tPostgres,ashwinstar/gpdb,lisakowen/gpdb,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,50wu/gpdb,greenplum-db/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,postmind-net/postgres-xl,pavanvd/postgres-xl,zeroae/postgres-xl,yazun/postgres-xl,ashwinstar/gpdb,postmind-net/postgres-xl,50wu/gpdb,tpostgres-projects/tPostgres,ovr/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,yazun/postgres-xl,lisakowen/gpdb,jmcatamney/gpdb,yazun/postgres-xl,oberstet/postgres-xl,ovr/postgres-xl,arcivanov/postgres-xl,kmjungersen/PostgresXL,jmcatamney/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,snaga/postgres-xl,Postgres-XL/Postgres-XL,snaga/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,yazun/postgres-xl,postmind-net/postgres-xl,pavanvd/postgres-xl,zeroae/postgres-xl,kmjungersen/PostgresXL,ovr/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,adam8157/gpdb,postmind-net/postgres-xl,50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,ovr/postgres-xl,snaga/postgres-xl,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,tpostgres-projects/tPostgres,oberstet/postgres-xl,ovr/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,snaga/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,snaga/postgres-xl,xinzweb/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,50wu/gpdb,kmjungersen/PostgresXL,pavanvd/postgres-xl,lisakowen/gpdb,xinzweb/gpdb,arcivanov/postgres-xl,jmcatamney/gpdb,postmind-net/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb