pgsql: Fix WAL replay of truncate operations to cope with the

Log Message:
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Fix WAL replay of truncate operations to cope with the possibility that the
truncated relation was deleted later in the WAL sequence. Since replay
normally auto-creates a relation upon its first reference by a WAL log entry,
failure is seen only if the truncate entry happens to be the first reference
after the checkpoint we're restarting from; which is a pretty unusual case but
of course not impossible. Fix by making truncate entries auto-create like
the other ones do. Per report and test case from Dharmendra Goyal.

Modified Files:
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pgsql/src/backend/storage/smgr:
smgr.c (r1.104 -> r1.105)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/smgr.c.diff?r1=1.104&r2=1.105)