QXmlStream: sanitize input before building a QChar

The result of getChar() may be a combination of a token category
and a code unit, like `((LETTER << 16) | c)`. Constructing a
QChar out of it requires masking the category out. This was already
done in a few code paths but missing in others.

Change-Id: I186fb01973a1badd425b3a6811843d8a40b13a6a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/serialization/qxmlstream.cpp

@@ -1189,7 +1189,7 @@ inline int QXmlStreamReaderPrivate::fastScanLiteralContent()
                 putChar(c);
                 return n;
             }
-            textBuffer += QChar(c);
+            textBuffer += QChar(ushort(c));
             ++n;
         }
     }
@@ -1345,7 +1345,7 @@ inline int QXmlStreamReaderPrivate::fastScanName(int *prefix)
             }
             Q_FALLTHROUGH();
         default:
-            textBuffer += QChar(c);
+            textBuffer += QChar(ushort(c));
             ++n;
         }
     }