smart_pkt: treat empty packet lines as error

The Git protocol does not specify what should happen in the case
of an empty packet line (that is a packet line "0004"). We
currently indicate success, but do not return a packet in the
case where we hit an empty line. The smart protocol was not
prepared to handle such packets in all cases, though, resulting
in a `NULL` pointer dereference.

Fix the issue by returning an error instead. As such kind of
packets is not even specified by upstream, this is the right
thing to do.

src/transports/smart_pkt.c

@@ -437,13 +437,13 @@ int git_pkt_parse_line(
 
 	line += PKT_LEN_SIZE;
 	/*
-	 * TODO: How do we deal with empty lines? Try again? with the next
-	 * line?
+	 * The Git protocol does not specify empty lines as part
+	 * of the protocol. Not knowing what to do with an empty
+	 * line, we should return an error upon hitting one.
 	 */
 	if (len == PKT_LEN_SIZE) {
-		*head = NULL;
-		*out = line;
-		return 0;
+		giterr_set_str(GITERR_NET, "Invalid empty packet");
+		return GIT_ERROR;
 	}
 
 	if (len == 0) { /* Flush pkt */

src/transports/smart_protocol.c

@@ -759,14 +759,6 @@ static int add_push_report_sideband_pkt(git_push *push, git_pkt_data *data_pkt,
 		line_len -= (line_end - line);
 		line = line_end;
 
-		/* When a valid packet with no content has been
-		 * read, git_pkt_parse_line does not report an
-		 * error, but the pkt pointer has not been set.
-		 * Handle this by skipping over empty packets.
-		 */
-		if (pkt == NULL)
-			continue;
-
 		error = add_push_report_pkt(push, pkt);
 
 		git_pkt_free(pkt);
@@ -821,9 +813,6 @@ static int parse_report(transport_smart *transport, git_push *push)
 
 		error = 0;
 
-		if (pkt == NULL)
-			continue;
-
 		switch (pkt->type) {
 			case GIT_PKT_DATA:
 				/* This is a sideband packet which contains other packets */