fix(groups): check for label in timer audio playing timer callback

If peer quits the group and they were just playing audio, their label will be removed but their timer will still be running. Unguarded update of label causes segfault.

Fix #5511

src/widget/form/groupchatform.cpp

@@ -376,8 +376,10 @@ void GroupChatForm::peerAudioPlaying(ToxPk peerPk)
             if (netcam) {
                 static_cast<GroupNetCamView*>(netcam)->removePeer(peerPk);
             }
-
-            peerLabels[peerPk]->setProperty("playingAudio", LABEL_PEER_NOT_PLAYING_AUDIO);
+            auto it = peerLabels.find(peerPk);
+            if (it != peerLabels.end()) {
+                peerLabels[peerPk]->setProperty("playingAudio", LABEL_PEER_NOT_PLAYING_AUDIO);
+            }
             delete peerAudioTimers[peerPk];
             peerAudioTimers[peerPk] = nullptr;
         });

src/widget/widget.cpp

@@ -1818,7 +1818,6 @@ void Widget::onGroupPeerAudioPlaying(int groupnumber, ToxPk peerPk)
     }
 
     auto form = groupChatForms[g->getId()].data();
-    // TODO(sudden6): switch to ToxPk here
     form->peerAudioPlaying(peerPk);
 }