Fix data race when using wxTheApp::m_handlersWithPendingEvents

Appending to the vector from another thread could reallocate it, making
access to its first element from the main thread invalid and resulting
in probably very hard to diagnose crashes.

Fix this by storing the handler in a local variable before releasing the
critical section protecting the vector. This still relies on the handler
not being deleted, but this was the case before too and shouldn't happen
and, most importantly, there doesn't seem to be anything we can do about
it if it does.

Thanks TSAN for the warning.

src/common/appbase.cpp

@@ -556,15 +556,18 @@ void wxAppConsoleBase::ProcessPendingEvents()
         // from it when they don't have any more pending events
         while (!m_handlersWithPendingEvents.IsEmpty())
         {
-            // In ProcessPendingEvents(), new handlers might be added
-            // and we can safely leave the critical section here.
-            wxLEAVE_CRIT_SECT(m_handlersWithPendingEventsLocker);
-
             // NOTE: we always call ProcessPendingEvents() on the first event handler
             //       with pending events because handlers auto-remove themselves
             //       from this list (see RemovePendingEventHandler) if they have no
             //       more pending events.
-            m_handlersWithPendingEvents[0]->ProcessPendingEvents();
+            wxEvtHandler* const handler = m_handlersWithPendingEvents[0];
+
+            // In ProcessPendingEvents(), new handlers might be added
+            // and we can safely leave the critical section here as we're not
+            // accessing m_handlersWithPendingEvents while we don't hold it.
+            wxLEAVE_CRIT_SECT(m_handlersWithPendingEventsLocker);
+
+            handler->ProcessPendingEvents();
 
             wxENTER_CRIT_SECT(m_handlersWithPendingEventsLocker);
         }