Refs #9761. Check to see if m_checkAvailabilityTimerId=0 before killi…

…ng it.

Prevents the frontend issuing the following error message.
"Application asked to unregister timer 0x0 which is not registered in this
thread. Fix application."

Thanks to Jonatan for the patch.

Stuart

mythtv/programs/mythfrontend/playbackboxhelper.cpp

@@ -104,7 +104,7 @@ AvailableStatusType PBHEventHandler::CheckAvailability(const QStringList &slist)
             m_checkAvailability.find(evinfo.MakeUniqueKey());
         if (it != m_checkAvailability.end())
             m_checkAvailability.erase(it);
-        if (m_checkAvailability.empty())
+        if (m_checkAvailability.empty() && m_checkAvailabilityTimerId)
             killTimer(m_checkAvailabilityTimerId);
     }