pgsql-server: Fix failure to advance nextXID beyond subtransactions

Log Message:
-----------
Fix failure to advance nextXID beyond subtransactions whose XIDs appear
only within COMMIT or ABORT records.

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
xact.c (r1.184 -> r1.185)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.184&r2=1.185)