Fix portalmem.c to avoid keeping a dangling pointer to a cached plan list

Fix portalmem.c to avoid keeping a dangling pointer to a cached plan list
after it's released its reference count for the cached plan.  There are
code paths that might try to examine the plan list before noticing that
the portal is already in aborted state.  Report and diagnosis by Tatsuo
Ishii, though this isn't exactly his proposed patch.