pvr/CGUIWindowPVRBase: check m_channelGroup before dereferencing it

Fixes several nullptr dereference bugs which could occur after the
VNSI connection was closed by VDR, e.g.:

```
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __GI___pthread_mutex_lock (mutex=0xf0) at ../nptl/pthread_mutex_lock.c:65
65	../nptl/pthread_mutex_lock.c: No such file or directory.
[Current thread is 1 (Thread 0x7fabcc406980 (LWP 686))]
(gdb) bt
#0  __GI___pthread_mutex_lock (mutex=0xf0) at ../nptl/pthread_mutex_lock.c:65
#1  0x000055afd0fd788c in (anonymous namespace)::CRecursiveMutex::lock (this=0xf0)
    at xbmc/threads/platform/RecursiveMutex.h:45
#2  0x000055afd0fd8978 in (anonymous namespace)::CountingLockable<XbmcThreads::CRecursiveMutex>::lock (this=0xf0)
    at xbmc/threads/Lockables.h:63
#3  0x000055afd0fd87c6 in (anonymous namespace)::UniqueLock<CCriticalSection>::UniqueLock (this=0x7ffd221f0450, lockable=...)
    at xbmc/threads/Lockables.h:132
#4  0x000055afd0ff3fed in CSingleLock::CSingleLock (this=0x7ffd221f0450, cs=...) at xbmc/threads/SingleLock.h:39
#5  0x000055afd1bc6147 in PVR::CPVRChannelGroup::GroupName (this=0x0) at xbmc/pvr/channels/PVRChannelGroup.cpp:1142
#6  0x000055afd1b31555 in PVR::CGUIWindowPVRBase::UpdateButtons (this=0x55afd6d030b0)
    at xbmc/pvr/windows/GUIWindowPVRBase.cpp:480
#7  0x000055afd1b39898 in PVR::CGUIWindowPVRRecordingsBase::UpdateButtons (this=0x55afd6d030b0)
    at xbmc/pvr/windows/GUIWindowPVRRecordings.cpp:187
#8  0x000055afd1459dc7 in CGUIMediaWindow::Update (this=0x55afd6d030b0, strDirectory=..., updateFilterPath=true)
    at xbmc/windows/GUIMediaWindow.cpp:905
```

xbmc/pvr/windows/GUIWindowPVRBase.cpp

@@ -194,8 +194,10 @@ bool CGUIWindowPVRBase::OnAction(const CAction &action)
     case ACTION_NEXT_CHANNELGROUP:
     {
       // switch to next or previous group
-      const CPVRChannelGroupPtr channelGroup = GetChannelGroup();
-      SetChannelGroup(action.GetID() == ACTION_NEXT_CHANNELGROUP ? channelGroup->GetNextGroup() : channelGroup->GetPreviousGroup());
+      if (const CPVRChannelGroupPtr channelGroup = GetChannelGroup())
+      {
+        SetChannelGroup(action.GetID() == ACTION_NEXT_CHANNELGROUP ? channelGroup->GetNextGroup() : channelGroup->GetPreviousGroup());
+      }
       return true;
     }
     case ACTION_MOVE_RIGHT:
@@ -377,7 +379,10 @@ bool CGUIWindowPVRBase::OpenChannelGroupSelectionDialog(void)
   dialog->SetHeading(CVariant{g_localizeStrings.Get(19146)});
   dialog->SetItems(options);
   dialog->SetMultiSelection(false);
-  dialog->SetSelected(GetChannelGroup()->GroupName());
+  if (const CPVRChannelGroupPtr channelGroup = GetChannelGroup())
+  {
+    dialog->SetSelected(channelGroup->GroupName());
+  }
   dialog->Open();
 
   if (!dialog->IsConfirmed())
@@ -480,7 +485,11 @@ void CGUIWindowPVRBase::UpdateButtons(void)
   CGUIMediaWindow::UpdateButtons();
 
   const CPVRChannelGroupPtr channelGroup = GetChannelGroup();
-  SET_CONTROL_LABEL(CONTROL_BTNCHANNELGROUPS, g_localizeStrings.Get(19141) + ": " + channelGroup->GroupName());
+  if (channelGroup)
+  {
+    SET_CONTROL_LABEL(CONTROL_BTNCHANNELGROUPS, g_localizeStrings.Get(19141) + ": " + channelGroup->GroupName());
+  }
+
   m_channelGroupsSelector->SelectChannelGroup(channelGroup);
 }