ico image handler: check for out of range image size

Make the decoder fail early to avoid spending time and memory on
attempting to decode a corrupt image file.

Change-Id: I598db817c387867a449040f5be5427c8b8746483
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/plugins/imageformats/ico/qicohandler.cpp

@@ -506,6 +506,8 @@ QImage ICOReader::iconAt(int index)
                 icoAttrib.h = iconEntry.bHeight;
                 if (icoAttrib.h == 0) // means 256 pixels
                     icoAttrib.h = header.biHeight/2;
+                if (icoAttrib.w > 256 || icoAttrib.h > 256) // Max ico size
+                    return img;
 
                 QImage::Format format = QImage::Format_ARGB32;
                 if (icoAttrib.nbits == 24)