pgsql-server: Tweak md.c logic to cope with the situation where WAL

Log Message:
-----------
Tweak md.c logic to cope with the situation where WAL replay tries to
write into a high-numbered segment of a relation that was later deleted.
We need to temporarily recreate missing segment files, instead of
failing.

Modified Files:
--------------
    pgsql-server/src/backend/storage/smgr:
        md.c (r1.111 -> r1.112)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/smgr/md.c.diff?r1=1.111&r2=1.112)