Guard against null plan pointer in CachedPlanIsSimplyValid().

Guard against null plan pointer in CachedPlanIsSimplyValid().

If both the passed-in plan pointer and plansource->gplan are
NULL, CachedPlanIsSimplyValid would think that the plan pointer
is possibly-valid and try to dereference it.  For the one extant
call site in plpgsql, this situation doesn't normally happen
which is why we've not noticed. However, it appears to be possible
if the previous use of the cached plan failed, as per report from
Justin Pryzby.  Add an extra check to prevent crashing.
Back-patch to v13 where this code was added.

Discussion: https://postgr.es/m/ZLlV+STFz1l/WhAQ@telsasoft.com