[orchagent]: Fix crash during orchagent process exit (sonic-net#974)

Avoid crash due to accessing deleted pointer, during process exit.
In this case, FdbOrch was calling detach on PortsOrch which is already deleted
and the memory got returned to Kernel, which resulted in kernel exception.

orchagent/orchdaemon.cpp

@@ -45,8 +45,20 @@ OrchDaemon::OrchDaemon(DBConnector *applDb, DBConnector *configDb, DBConnector *
 OrchDaemon::~OrchDaemon()
 {
     SWSS_LOG_ENTER();
-    for (Orch *o : m_orchList)
-        delete(o);
+
+    /*
+     * Some orchagents call other agents in their destructor.
+     * To avoid accessing deleted agent, do deletion in reverse order.
+     * NOTE: This is stil not a robust solution, as order in this list
+     *       does not strictly match the order of construction of agents. 
+     * For a robust solution, first some cleaning/house-keeping in 
+     * orchagents management is in order.
+     * For now it fixes, possible crash during process exit.
+     */
+    auto it = m_orchList.rbegin();
+    for(; it != m_orchList.rend(); ++it) {
+        delete(*it);
+    }
 }
 
 bool OrchDaemon::init()