Set the size of the buffer so it is big enough to hold the contents

This will prevent a crash later on when it tries to assign to an index
in a QString that has not been allocated.

Fixes: QTBUG-81950
Change-Id: Ia0b5648a18f15594eeca07d234bedadcfeb266ac
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/widgets/qlcdnumber.cpp

@@ -713,7 +713,7 @@ void QLCDNumber::paintEvent(QPaintEvent *)
 void QLCDNumberPrivate::internalSetString(const QString& s)
 {
     Q_Q(QLCDNumber);
-    QString buffer;
+    QString buffer(ndigits, QChar());
     int i;
     int len = s.length();
     QBitArray newPoints(ndigits);