Fix segfault reported by ClusterFuzz.

Source/core/dom/FullscreenElementStack.cpp

Index: Source/core/dom/FullscreenElementStack.cpp
diff --git a/Source/core/dom/FullscreenElementStack.cpp b/Source/core/dom/FullscreenElementStack.cpp
index 1e90d92b3cd4f5e9ee6201d0cdcf689628398cae..8415b3d14d72d3695d038be46e3fb31c34de0a2f 100644
--- a/Source/core/dom/FullscreenElementStack.cpp
+++ b/Source/core/dom/FullscreenElementStack.cpp
@@ -61,12 +61,12 @@ static bool fullscreenIsSupported(const Document& document)
 {
     // Fullscreen is supported if there is no previously-established user preference,
     // security risk, or platform limitation.
-    return document.settings()->fullscreenSupported();
+    return !document.settings() || document.settings()->fullscreenSupported();
 }
 
 static bool fullscreenIsSupported(const Document& document, const Element& element)
 {
-    if (document.settings()->disallowFullscreenForNonMediaElements() && !isHTMLMediaElement(element))
+    if (!document.settings() || (document.settings()->disallowFullscreenForNonMediaElements() && !isHTMLMediaElement(element)))
         return false;
     return fullscreenIsSupported(document);
 }