Fix possible assertion violation in PeerListBox.

Very long stack in crash reports leads to something like that:
- PeerListBox::prepare
- PeerListBox::createMultiSelect
- PeerListBox::updateScrollSkips
- BoxContent::setInnerTopSkip
- _scroll->scrollToY
- sendSynteticMouseEvent
- ChatHelpers::TabbedPanel::showAnimated
- QWidget::render
- QWidgetPrivate::sendPendingMoveAndResizeEvents
- PeerListBox::resizeEvent
- _select->resizeToWidth(0)
- MultiSelect::Inner::computeItemsGeometry(0)

Workaround:
- Don't scrollToY if PeerListBox width was not yet set.
- Initial _scrollBottomFixed is false (at first createMultiSelect).

Telegram/SourceFiles/boxes/abstract_box.cpp

@@ -148,7 +148,7 @@ void BoxContent::setInnerTopSkip(int innerTopSkip, bool scrollBottomFixed) {
 	if (_innerTopSkip != innerTopSkip) {
 		auto delta = innerTopSkip - _innerTopSkip;
 		_innerTopSkip = innerTopSkip;
-		if (_scroll) {
+		if (_scroll && width() > 0) {
 			auto scrollTopWas = _scroll->scrollTop();
 			updateScrollAreaGeometry();
 			if (scrollBottomFixed) {

Telegram/SourceFiles/boxes/peer_list_box.h

@@ -807,6 +807,6 @@ private:
 
 	std::unique_ptr<PeerListController> _controller;
 	base::lambda<void(PeerListBox*)> _init;
-	bool _scrollBottomFixed = true;
+	bool _scrollBottomFixed = false;
 
 };