Fix (unlikely) truncation in write_xbm_image()

The usual qsizetype -> int problem. Fix by using auto.

Change-Id: Ie990dec39c8c33cdc055da6bfee451319825c934
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 0cef4ae6870fdd57fd908cdecde3c6c426b7321e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qxbmhandler.cpp

@@ -164,7 +164,7 @@ static bool write_xbm_image(const QImage &sourceImage, QIODevice *device, const
     int        h = image.height();
     int        i;
     const QByteArray s = fileName.toUtf8(); // get file base name
-    int        msize = s.size() + 100;
+    const auto msize = s.size() + 100;
     char *buf = new char[msize];
 
     qsnprintf(buf, msize, "#define %s_width %d\n", s.data(), w);