refs.c: fix fread error handling

fread returns the number of items read, with no special error return.

Commit 98f85ff (reflog: add for_each_reflog_ent_reverse() API -
2013-03-08) introduced a call to fread which checks for an error with
"nread < 0" which is tautological since nread is unsigned.  The correct
check in this case (which tries to read a single item) is "nread != 1".

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>