Fix konsole crashes when rearranging split views

I'll try to explain here what I saw in this bug.
Following the BUG instructions, drag the terminal[2] and drop
(here the "drop event" starts)
-> the dragged terminal "ViewSplitter" become scheduled for deletion,
but it will only happens when it is in event loop.
-> And of course it is not in event loop yet, it is still the
"drop event" and it call the "addTerminalDisplay" to add the dragged
 terminal to a "ViewSplitter".
-> This makes count() counts this scheduled for deletion "ViewSplitter"
 and goes to the wrong "if".
-> It adds the "ViewSplitter" with the terminals into a
"marked for deletion" one, finishes the "drop event" and enters
event loop (a crash without a memleak! everything in the tab
 is properly deleted).

BUG: 411741
BUG: 412596
FIXED-IN: 20.12

src/widgets/ViewSplitter.cpp

@@ -137,6 +137,10 @@ void ViewSplitter::childEvent(QChildEvent *event)
 
     if (event->removed()) {
         if (count() == 0) {
+            auto *parent_splitter = qobject_cast<ViewSplitter *>(parentWidget());
+            if (parent_splitter != nullptr) {
+                setParent(nullptr);
+            }
             deleteLater();
         }
         if (findChild<TerminalDisplay*>() == nullptr) {