pgsql: Fix thinko in pg_read_file: testing for negative result is not

Log Message:
-----------
Fix thinko in pg_read_file: testing for negative result is not the way
to determine whether fread() failed.

Modified Files:
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pgsql/src/backend/utils/adt:
genfile.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/genfile.c.diff?r1=1.7&r2=1.8)