fread does not return negative on error

size_t res cannot be less than 0. fread returns 0 on error.
Reported-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NRoel Kluin <roel.kluin@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

strbuf.c

@@ -260,7 +260,7 @@ size_t strbuf_fread(struct strbuf *sb, size_t size, FILE *f)
 	res = fread(sb->buf + sb->len, 1, size, f);
 	if (res > 0)
 		strbuf_setlen(sb, sb->len + res);
-	else if (res < 0 && oldalloc == 0)
+	else if (oldalloc == 0)
 		strbuf_release(sb);
 	return res;
 }