Avoid using the current frame as the previous in layout

When continuing a previous layout, we would in some cases add a wrong
margin because we would treat the current frame as preceding itself.

Task-number: QTBUG-42269
Change-Id: Idf00fadbed890ce34b073cf670cc9b8e1d4b0d5d
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/gui/text/qtextdocumentlayout.cpp

@@ -2432,7 +2432,7 @@ void QTextDocumentLayoutPrivate::layoutFlow(QTextFrame::Iterator it, QTextLayout
             ++it;
         } else {
             QTextFrame::Iterator lastIt;
-            if (!previousIt.atEnd())
+            if (!previousIt.atEnd() && previousIt != it)
                 lastIt = previousIt;
             previousIt = it;
             QTextBlock block = it.currentBlock();