Avoid changes of d->cString for TQString::shared_null to make the value reliable.

This precedes unnecessary allocations, potential use after free and crashes.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit 93058fb0a3)

src/tools/qstring.cpp

@@ -5991,6 +5991,14 @@ void QString::setSecurityUnPaged(bool lock) {
 */
 QCString QString::utf8() const
 {
+    if (!d->cString) {
+        d->cString = new QCString;
+    }
+    if(d == shared_null)
+    {
+        return *d->cString;
+    }
+
     int l = length();
     int rlen = l*3+1;
     QCString rstr(rlen);
@@ -6035,11 +6043,8 @@ QCString QString::utf8() const
  	++ch;
     }
     rstr.truncate( cursor - (uchar*)rstr.data() );
-    if (!d->cString) {
-        d->cString = new QCString;
-    }
     *d->cString = rstr;
-    return rstr;
+    return *d->cString;
 }
 
 static QChar *addOne(QChar *qch, QString &str)
@@ -6243,6 +6248,10 @@ QCString QString::local8Bit() const
     if (!d->cString) {
         d->cString = new QCString;
     }
+    if(d == shared_null)
+    {
+        return *d->cString;
+    }
 #ifdef QT_NO_TEXTCODEC
     *d->cString = QCString(latin1());
     return *d->cString;