pgsql-server: Add missing Assert to make TransactionIdDidAbort more

Log Message:
-----------
Add missing Assert to make TransactionIdDidAbort more consistent with
TransactionIdDidCommit.

Modified Files:
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pgsql-server/src/backend/access/transam:
transam.c (r1.58 -> r1.59)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/transam.c.diff?r1=1.58&r2=1.59)