Fix pfree crash in pg_get_role_ddl() and pg_get_database_ddl().

Fix pfree crash in pg_get_role_ddl() and pg_get_database_ddl().

DatumGetArrayTypeP() can return a pointer into the tuple when the
datum is stored as a short varlena, so pfree() on the result crashes.
Use DatumGetArrayTypePCopy() to always get a palloc'd copy.

Bug introduced in 76e514ebb4b and a4f774cf1c7.

Reported-by: Jeff Davis <pgsql@j-davis.com>
Author: Satya Narlapuram <satya.narlapuram@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDdWtv9PKtPZEokwGCNtbv4MVnfYw5wMZrsEj4xizSNe5Q@mail.gmail.com