Failures in allocations could lead to crashes with NULL pointer
dereferences .  Memory context TopMemoryContext is used instead to keep
alive the plans allocated in the session.  A more specific context could
be used here, but this is left for later.

Reported-by: Jian Zhang
Author: Michael Paquier
Reviewed-by: Tom Lane, Andres Freund
Discussion: https://postgr.es/m/16190-70181c803641c3dc@postgresql.org