session: Use session->thread_index to correctly retrieve the session

session: Use session->thread_index to correctly retrieve the session

For non-connected udp, when retrieving the subscriber session to send
the notification, it uses the current worker thread index whereas the
subscriber session is actually on the main thread. Using the worker
thread may cause a crash since the corresponding session may not be
valid in the worker thread context and even if it is valid, it is the
wrong session. This scenario is seen when the application forks
and adds subscribers to the worker thread session.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I236ee9d9ff9f3b2f7f9f8e782d70d1080aa1b627