Distinguish EOF from a read error when loading the relcache init file

Hi,

load_relcache_init_file() treats a zero-length fread() of the next
relation descriptor length as the end of pg_internal.init, which has
no record count.  fread() also returns zero when a read error
transfers no data.  In that case the file is accepted whenever the
nailed rel and index counts already match, so a partial cache can be
installed instead of bootstrapping it from the built-in descriptors.

Check ferror() before treating a zero-length read as the end of the
file.

The patch is attached.

Regards,
Evgeniy Gorbanyov