QXmlStreamReader: port readBufferPos from int to qsizetype

The variable represents an offset into the QString readBuffer data
member. Given a QString with more than 2Gi of characters, the variable
would overflow before we've consumed all of readBuffer's
contents.

Signed integer overflow is UB, so anything could happen.

Task-number: QTBUG-102465
Pick-to: 6.3 6.2
Change-Id: I05d35a5e7f02f05462b318c86b17fdab7f1afec6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qxmlstream_p.h

@@ -250,7 +250,7 @@ public:
     uchar firstByte;
     qint64 nbytesread;
     QString readBuffer;
-    int readBufferPos;
+    qsizetype readBufferPos;
     QXmlStreamSimpleStack<uint> putStack;
     struct Entity {
         Entity() = default;