chan_websocket: Add locking in send_event and check for NULL websocket handle.

On an outbound websocket connection, when the triggering caller hangs up,
webchan_hangup() closes the outbound websocket session and sets the websocket
session handle to NULL.  If the hangup happened in the tiny window between
opening the outbound websocket connection and before read_thread_handler()
was able to send the MEDIA_START message, it could segfault because the
websocket session handle was NULL.  If it didn't actually segfault, there was
also the possibility that the websocket instance wouldn't get cleaned up which
could also cause the channel snapshot to not get cleaned up.  That could
cause memory leaks and `core show channels` to list phantom WebSocket
channels.

To prevent the race, the send_event() macro now locks the websocket_pvt
instance and checks the websocket session handle before attempting to send
the MEDIA_START message.

Resolves: #1643
Resolves: #1645

channels/chan_websocket.c

@@ -416,7 +416,8 @@ static __attribute__ ((format (gnu_printf, 2, 3))) char *_create_event_ERROR(
 ({ \
 	int _res = -1; \
 	char *_payload = _create_event_ ## _event(_instance, ##__VA_ARGS__); \
-	if (_payload) { \
+	ao2_lock(instance); \
+	if (_payload && _instance->websocket) { \
 		_res = ast_websocket_write_string(_instance->websocket, _payload); \
 		if (_res != 0) { \
 			ast_log(LOG_ERROR, "%s: Unable to send event %s\n", \
@@ -427,6 +428,7 @@ static __attribute__ ((format (gnu_printf, 2, 3))) char *_create_event_ERROR(
 		}\
 		ast_free(_payload); \
 	} \
+	ao2_unlock(instance); \
 	(_res); \
 })