DRTVWR-476: Make ~LLEventPumps() call reset() on its way out.

~LLEventPumps() deletes every LLEventPump instance it created itself. However,
many classes themselves contain LLEventPump subclass instances. These are
registered with LLEventPumps without it managing their lifespan.

But LLEventPump::reset() frees the LLStandardSignal aka
boost::signals2::signal instance owned by the LLEventPump, perforce
disconnecting all current listeners and disabling the LLEventPump. Even though
the instance still exists, if someone subsequently calls post(), nothing will
happen -- which is better than control trying to reach a method of a deleted
object.

indra/llcommon/llevents.cpp

@@ -266,6 +266,9 @@ LLEventPumps::~LLEventPumps()
     {
         delete *mOurPumps.begin();
     }
+    // Reset every remaining registered LLEventPump subclass instance: those
+    // we DIDN'T instantiate using either make() or obtain().
+    reset();
 }
 
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