fix race condition when hangup happends after answer indication but before the session thread is started

src/include/private/switch_core_pvt.h

@@ -92,7 +92,9 @@ typedef enum {
 	SSF_NONE = 0,
 	SSF_DESTROYED = (1 << 0),
 	SSF_WARN_TRANSCODE = (1 << 1),
-	SSF_HANGUP = (1 << 2)
+	SSF_HANGUP = (1 << 2),
+	SSF_THREAD_STARTED = (1 << 3),
+	SSF_THREAD_RUNNING = (1 << 4)
 } switch_session_flag_t;
 
 
@@ -103,7 +105,6 @@ struct switch_core_session {
 	switch_endpoint_interface_t *endpoint_interface;
 	switch_size_t id;
 	switch_session_flag_t flags;
-	int thread_running;
 	switch_channel_t *channel;
 
 	switch_io_event_hooks_t event_hooks;

src/include/switch_core.h

@@ -479,6 +479,7 @@ SWITCH_DECLARE(void) switch_core_session_run(_In_ switch_core_session_t *session
   \param session the session on which to check
 */
 SWITCH_DECLARE(unsigned int) switch_core_session_running(_In_ switch_core_session_t *session);
+SWITCH_DECLARE(unsigned int) switch_core_session_started(_In_ switch_core_session_t *session);
 
 SWITCH_DECLARE(void *) switch_core_perform_permanent_alloc(_In_ switch_size_t memory, _In_z_ const char *file, _In_z_ const char *func, _In_ int line);