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.

Modified Files:
--------------
    pgsql/src/backend/commands:
        prepare.c (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.43&r2=1.44)
    pgsql/src/backend/tcop:
        postgres.c (r1.470 -> r1.471)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.470&r2=1.471)
    pgsql/src/include/commands:
        prepare.h (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/prepare.h.diff?r1=1.15&r2=1.16)