[fix] PicDocument: Pass a copy of image_bb in getCoverPageImage() (#4628)

Avoids a use-after-free in mupdf.scaleBlitBuffer

Fix koreader/koreader-base#821

Thanks for the hint, @poire-z ;).

* As @poire-z suggested, the original unscaled bb should probably be free'd.

frontend/document/picdocument.lua

@@ -43,7 +43,7 @@ end
 function PicDocument:getCoverPageImage()
     local first_page = self._document:openPage(1)
     if first_page.image_bb then
-        return first_page.image_bb
+        return first_page.image_bb:copy()
     end
     return nil
 end