Fix duplicate subtitle widget in group call panel

The _subtitle.create() rpl chain calls setupMembers() synchronously
during the FlatLabel constructor. setupMembers() triggers geometry
updates that re-enter refreshTitle() while object_ptr::create() has
not yet assigned the new widget to _subtitle, so a second subtitle
is created and left as an orphaned visible child.

Moving setupMembers() before _subtitle.create() ensures _members
exists before the rpl chain fires, preventing the re-entrancy.

Author: paterkleomenis

Telegram/SourceFiles/calls/group/calls_group_panel.cpp

@@ -2792,6 +2792,9 @@ void Panel::refreshTitle() {
 	}
 	refreshTitleGeometry();
 	if (!_subtitle && mode() == PanelMode::Default) {
+		if (!_members) {
+			setupMembers();
+		}
 		_subtitle.create(
 			widget(),
 			rpl::single(