pvr: Fix crash when opening PVROSDChannels while playing a recording

This fixes a crash when sending the action XBMC.ActivateWindow(PVROSDChannels)
to open the channel view while playing back a PVR recording.

 0  0x00007ffff0d2b037 in __GI_raise (sig=sig@entry=6)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
 1  0x00007ffff0d2e698 in __GI_abort () at abort.c:90
 2  0x00007ffff0d23e03 in __assert_fail_base (
    fmt=0x7ffff0e7a578 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
    assertion=assertion@entry=0x10e09c9 "px != 0",
    file=file@entry=0x10e06f8 "/usr/include/boost/smart_ptr/shared_ptr.hpp",
    line=line@entry=424,
    function=function@entry=0x119ede0
      <_ZZNK5boost10shared_ptrIN3PVR11CPVRChannelEEptEvE19__PRETTY_FUNCTION__>
      "T* boost::shared_ptr<T>::operator->() const [with T = PVR::CPVRChannel]")
      at assert.c:92
 3  0x00007ffff0d23eb2 in __GI___assert_fail (assertion=0x10e09c9 "px != 0",
    file=0x10e06f8 "/usr/include/boost/smart_ptr/shared_ptr.hpp", line=424,
    function=0x119ede0
       <_ZZNK5boost10shared_ptrIN3PVR11CPVRChannelEEptEvE19__PRETTY_FUNCTION__>
      "T* boost::shared_ptr<T>::operator->() const [with T = PVR::CPVRChannel]")
      at assert.c:101
 4  0x00000000006db05e in boost::shared_ptr<PVR::CPVRChannel>::operator-> (
    this=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:424
 5  0x000000000103af4d in PVR::CGUIDialogPVRChannelsOSD::Update (
    this=this@entry=0x1fe2a40) at GUIDialogPVRChannelsOSD.cpp:183
 6  0x000000000103b2fd in PVR::CGUIDialogPVRChannelsOSD::OnInitWindow (
    this=0x1fe2a40) at GUIDialogPVRChannelsOSD.cpp:105
 7  0x000000000080e3f9 in CGUIWindow::OnMessage (this=this@entry=0x1fe2a40,
    message=...) at GUIWindow.cpp:546
 8  0x00000000007f25ab in CGUIDialog::OnMessage (this=0x1fe2a40, message=...)
    at GUIDialog.cpp:103
 9  0x00000000007f222e in CGUIDialog::DoModal_Internal (this=0x1fe2a40,
    iWindowID=10610, param=...) at GUIDialog.cpp:185
 10 0x00000000008145ea in CGUIWindowManager::ActivateWindow_Internal (
    this=this@entry=0x16e5df0, iWindowID=iWindowID@entry=10610, params=...,
    swappingWindows=swappingWindows@entry=false) at GUIWindowManager.cpp:405
 11 0x000000000081496f in CGUIWindowManager::ActivateWindow (this=0x16e5df0,
    iWindowID=iWindowID@entry=10610, params=...,
    swappingWindows=swappingWindows@entry=false) at GUIWindowManager.cpp:353
 12 0x0000000000ae56ac in CBuiltins::Execute (execString=...)
    at Builtins.cpp:394
 13 0x0000000000bda289 in CApplication::OnAction (this=this@entry=0x16e1070,
    action=...) at Application.cpp:2502
 14 0x0000000000d78e08 in EVENTSERVER::CEventServer::ExecuteNextAction (
    this=this@entry=0x2c3d9c0) at EventServer.cpp:353
 15 0x0000000000bdc9ba in CApplication::ProcessEventServer (
    this=this@entry=0x16e1070, frameTime=frameTime@entry=0.0400000028)
    at Application.cpp:3039
 16 0x0000000000bdd326 in CApplication::FrameMove (this=0x16e1070,
    processEvents=<optimized out>, processGUI=<optimized out>)
    at Application.cpp:2838
 17 0x0000000000c521f2 in CXBApplicationEx::Run (this=this@entry=0x16e1070)
    at XBApplicationEx.cpp:140
 18 0x000000000071d5d7 in XBMC_Run (renderGUI=<optimized out>) at xbmc.cpp:69
 19 0x00000000006dba7e in main (argc=1, argv=0x7fffffffdff8)
    at xbmc/main/main.cpp:76