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:
----
REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/commands:
        prepare.c (r1.42 -> r1.42.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.42&r2=1.42.2.1)
    pgsql/src/backend/tcop:
        postgres.c (r1.468.2.2 -> r1.468.2.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.468.2.2&r2=1.468.2.3)
    pgsql/src/include/commands:
        prepare.h (r1.14 -> r1.14.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/prepare.h.diff?r1=1.14&r2=1.14.2.1)