pgsql: Defend against crash while processing Describe Statement or

Log Message:
-----------
Defend against crash while processing Describe Statement or Describe Portal
messages, when client attempts to execute these outside a transaction (start
one) or in a failed transaction (reject message, except for COMMIT/ROLLBACK
statements which we can handle).  Per report from Francisco Figueiredo Jr.

Tags:
----
REL7_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/commands:
        prepare.c (r1.23.4.2 -> r1.23.4.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.23.4.2&r2=1.23.4.3)
    pgsql/src/backend/tcop:
        postgres.c (r1.375.2.4 -> r1.375.2.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.375.2.4&r2=1.375.2.5)
    pgsql/src/include/commands:
        prepare.h (r1.8 -> r1.8.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/prepare.h.diff?r1=1.8&r2=1.8.4.1)