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_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/commands:
        prepare.c (r1.36 -> r1.36.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.36&r2=1.36.4.1)
    pgsql/src/backend/tcop:
        postgres.c (r1.440.4.4 -> r1.440.4.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.440.4.4&r2=1.440.4.5)
    pgsql/src/include/commands:
        prepare.h (r1.13 -> r1.13.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/prepare.h.diff?r1=1.13&r2=1.13.4.1)