Fix rendering error in some animated gifs

The code for handling of the rarely used "restore to previous" frame
disposal option would mistakenly clear the entire existing frame. It
looks like this was a copy/paste mistake in the code; the intention
obviously was to clear the newly created backingstore instead.

This fixes the rendering of
http://media1.giphy.com/media/9Jevgp0CgvAnm/giphy.gif

Task-number: QTBUG-55141
Change-Id: I4d344b7733edd62346656154e215c21727f9b6bb
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/image/qgifhandler.cpp

@@ -409,7 +409,7 @@ int QGIFFormat::decode(QImage *image, const uchar *buffer, int length,
                         backingstore = QImage(qMax(backingstore.width(), w),
                                               qMax(backingstore.height(), h),
                                               QImage::Format_RGB32);
-                        memset(bits, 0, image->byteCount());
+                        memset(backingstore.bits(), 0, backingstore.byteCount());
                     }
                     const int dest_bpl = backingstore.bytesPerLine();
                     unsigned char *dest_data = backingstore.bits();