Fix for bug 407053 ("Assertion failure: (cx)->requestDepth || (cx)->thread == (cx)->runtime->gcThread" with document.execCommand('copy', ...)). r/sr=jst, a=sicking.

content/html/document/src/nsHTMLDocument.cpp

@@ -4332,6 +4332,11 @@ nsHTMLDocument::DoClipboardSecurityCheck(PRBool aPaste)
   if (stack) {
     JSContext *cx = nsnull;
     stack->Peek(&cx);
+    if (!cx) {
+      return NS_OK;
+    }
+
+    JSAutoRequest ar(cx);
 
     NS_NAMED_LITERAL_CSTRING(classNameStr, "Clipboard");