pgsql: Fix coredump during replay WAL after crash.

Log Message:
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Fix coredump during replay WAL after crash. Change entrySplitPage() to prevent
usage of any information from system catalog, because it could be called during
replay of WAL.

Per bug report from Craig McElroy <craig.mcelroy@contegix.com>. Patch doesn't
change on-disk storage.

Modified Files:
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    pgsql/src/backend/access/gin:
        ginentrypage.c (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginentrypage.c?r1=1.9&r2=1.10)