Don't crash if we get a native DOM element back. sr=shaver

extensions/webdav/src/nsWebDAVUtils.cpp

@@ -47,6 +47,7 @@
 #include "nsIDOMNodeList.h"
 #include "nsIDOMDocument.h"
 #include "nsIDOMParser.h"
+#include "nsContentErrors.h"
 
 #if defined(PR_LOGGING)
 PRLogModuleInfo *gDAVLog = nsnull;
@@ -66,6 +67,10 @@ NS_WD_GetElementByTagName(nsIDOMElement *parentElt, const nsAString &tagName,
     rv = list->Item(0, getter_AddRefs(node));
     NS_ENSURE_SUCCESS(rv, rv);
 
+    if (!node) {
+        return NS_CONTENT_ATTR_NO_VALUE;
+    }
+
     return CallQueryInterface(node, elt);
 }