QXmlStreamReader: Don't resize readBuffer to a size it already has

Resizing it to 0 will cause it to allocate memory. This will then
cause append() to copy the data from the other string instead of
using copy on write.

Task-number: oss-fuzz-24347
Pick-to: 5.12 5.15
Change-Id: I581bd109f9b973e1c70b7b41b1f610a2ad5725b8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qxmlstream.cpp

@@ -1484,7 +1484,8 @@ uint QXmlStreamReaderPrivate::getChar_helper()
     const int BUFFER_SIZE = 8192;
     characterOffset += readBufferPos;
     readBufferPos = 0;
-    readBuffer.resize(0);
+    if (readBuffer.size())
+        readBuffer.resize(0);
     if (decoder.isValid())
         nbytesread = 0;
     if (device) {