always check for NULL return from packet_read_line()

The packet_read_line() function will die if it sees any
protocol or socket errors. But it will return NULL for a
flush packet; some callers which are not expecting this may
dereference NULL if they get an unexpected flush. This would
involve the other side breaking protocol, but we should
flag the error rather than segfault.
Signed-off-by: NJon Simons <jon@jonsimons.org>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote-curl.c

@@ -339,6 +339,8 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		 * pkt-line matches our request.
 		 */
 		line = packet_read_line_buf(&last->buf, &last->len, NULL);
+		if (!line)
+			die("invalid server response; expected service, got flush packet");
 
 		strbuf_reset(&exp);
 		strbuf_addf(&exp, "# service=%s", service);

send-pack.c

@@ -137,6 +137,8 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struc
 static int receive_unpack_status(int in)
 {
 	const char *line = packet_read_line(in, NULL);
+	if (!line)
+		return error(_("unexpected flush packet while reading remote unpack status"));
 	if (!skip_prefix(line, "unpack ", &line))
 		return error(_("unable to parse remote unpack status: %s"), line);
 	if (strcmp(line, "ok"))