Fix "NOTIFY received when line is idle" deadlock

This can easily be triggered by starting a blind transfer, and hanging
up before the other end has accepted/rejected it, due to the following
path:

- recvd_notify() acquires a read lock on lines_mtx
- cleanup_dead_lines() is called
- A write lock on lines_mtx is acquired

1 files changed, 1 insertions, 1 deletions

diff --git a/src/phone.cpp b/src/phone.cpp
index a609bef..c4e515f 100644
--- a/src/phone.cpp
+++ b/src/phone.cpp
@@ -1704,7 +1704,7 @@ void t_phone::recvd_notify(t_request *r, t_tid tid) {
 	}
 
 	// REFER notification
-	t_rwmutex_reader x(lines_mtx);
+	t_rwmutex_future_writer x(lines_mtx);
 	for (unsigned short i = 0; i < lines.size(); i++) {
 		if (lines[i]->match(r)) {
 			lines[i]->recvd_notify(r, tid);