res/res_pjsip: Fix crash due to misuse of session->media between threads.

This patch makes sure that thread running ast_taskprocessor_execute
cannot suddenly dispose the session->media object making the other
threads (running pbx_thread / bridge_channel_ind_thread) crash when they
try to access the pointer to invalid memory. We were experiencing a crash due
to a misuse of session->media container between threads running
(bridge_channel_ind_thread/pbx_thread) and the thread running
ast_taskprocessor_execute. Depending on the SIP flow (during a disconnection)
and the threads' code path, the session->media container was being destroyed
(and set to NULL) by the thread running ast_taskprocessor_execute while the
thread running t38_framehook_read was still referring to it.
Now res_pjsip_t38 is referring a session_media in a datastore.

ASTERISK-28156

Change-Id: Ia92e2389b8d804bf205473e92ec06217e87ce237