WebKitGTK+ 2.21.3 pageURL Mishandling Denial Of Service

webkitFaviconDatabaseSetIconForPageURL and webkitFaviconDatabaseSetIconURLForPageURL in UIProcess/API/glib/WebKitFaviconDatabase.cpp in WebKit, as used in WebKitGTK+ through 2.21.3, mishandle an unset pageURL, leading to an application crash.