fixed: switch channels using keypad while watching slingbox

This is an attempt to fix #14961. A user test build will be created here: http://jenkins.xbmc.org/job/XBMC-WIN-32/462/console
@jmarshallnz: I'm unsure if the case iChannelNumber > 0 is valid for slingbox and other file system implementations as well or if we should send ACTION_CHANNEL_SWITCH on REMOTE_0 for them as well.