refs.c: fix fread error handling

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

Commit 98f85ff4 (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: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.c

@@ -2357,7 +2357,7 @@ int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void
 			return error("cannot seek back reflog for %s: %s",
 				     refname, strerror(errno));
 		nread = fread(buf, cnt, 1, logfp);
-		if (nread < 0)
+		if (nread != 1)
 			return error("cannot read %d bytes from reflog for %s: %s",
 				     cnt, refname, strerror(errno));
 		pos -= cnt;