MT#65511 AmB2BSession: `media_session` clean in destructor

Normal shutdown already triggers the `clearRtpReceiverRelay()`
in `AmB2BSession::onBeforeDestroy()`, so the destructor should
actually normally see `media_session` ptr nulled.

But it seems in some of the fork/transfer related scenarios
this doesn't happen.

So adding the defensive clean-up on the same manner
the destructor of the `CallLeg` has (it uses the `releaseMediaSession()`,
which is essentially the same: `stop()`; `releaseReference()` and NULLing),
makes it consistent with these other_legs cleanup.

The main reason is caused by a call leg (`AmB2BSession`), which was freed
while the `AmB2BMedia::a` / `AmB2BMedia::b` still points to it.
In this case we can crash during the media-thread DTMF processing.

This fix is a hardening rather than a fix of the root cause.
The root cause still is related to the cleanup path:
certain call leg gets torn down through the path that skips (or maybe even races)
the normal `onBeforeDestroy()`/`clearRtpReceiverRelay()` cleanup,
leaving its shared `AmB2BMedia` with a dangling `a` or `b` pointer.
Meanwhile the `a` / `b` are raw, non-owned pointers, which may remain dangling.

Change-Id: Ia1ee42a27014d2086b37a787f8555e6d7681d4a8

core/AmB2BSession.cpp

@@ -127,6 +127,17 @@ AmB2BSession::~AmB2BSession()
 {
   ILOG_DLG(L_DBG, "relayed_req.size() = %zu\n",relayed_req.size());
 
+  /* defensive release of the primary media_session, for the case when
+   * some fork/transfer path forgets to call clearRtpReceiverRelay(),
+   * the media_session ptr still gets nulled out before the object's memory is released.
+   */
+  if (media_session) {
+    ILOG_DLG(L_ERR, "This media_session was not released before AmB2BSession destruction! Do it now.\n");
+    media_session->stop(a_leg);
+    media_session->releaseReference();
+    media_session = NULL;
+  }
+
   map<int,AmSipRequest>::iterator it = recvd_req.begin();
   ILOG_DLG(L_DBG, "recvd_req.size() = %zu\n",recvd_req.size());
   for(;it != recvd_req.end(); ++it){
@@ -137,7 +148,7 @@ AmB2BSession::~AmB2BSession()
     delete subs;
 }
 
-void AmB2BSession::set_sip_relay_only(bool r) { 
+void AmB2BSession::set_sip_relay_only(bool r) {
   if (!getLocalTag().empty())
     ILOG_DLG(L_DBG, "Set sip_relay_only=%s for local_tag '%s'\n", (r ? "true" : "false"), getLocalTag().c_str());