Fix warning about out of bounds access in QString::operator[]

Fixes runtime warnings that got triggered by change
c2d2757bcc.

Change-Id: I50620b179de8608f45d6f2ef053eeb8b1e10ae43
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/text/qtextodfwriter.cpp

@@ -358,7 +358,7 @@ void QTextOdfWriter::writeBlock(QXmlStreamWriter &writer, const QTextBlock &bloc
         int precedingSpaces = 0;
         int exportedIndex = 0;
         for (int i=0; i <= fragmentText.count(); ++i) {
-            QChar character = fragmentText[i];
+            QChar character = (i == fragmentText.count() ? QChar() : fragmentText.at(i));
             bool isSpace = character.unicode() == ' ';
 
             // find more than one space. -> <text:s text:c="2" />