check for null node to prevent monkey crash

Change-Id: I11625535de5e18f94d293081416eaa9bfa0a8c3c
http://b/2492244
diff --git a/WebKit/android/jni/WebViewCore.cpp b/WebKit/android/jni/WebViewCore.cpp
index c267b95..53f2b24 100644
--- a/WebKit/android/jni/WebViewCore.cpp
+++ b/WebKit/android/jni/WebViewCore.cpp
@@ -1288,7 +1288,7 @@
 WebCore::String WebViewCore::requestLabel(WebCore::Frame* frame,
         WebCore::Node* node)
 {
-    if (CacheBuilder::validNode(m_mainFrame, frame, node)) {
+    if (node && CacheBuilder::validNode(m_mainFrame, frame, node)) {
         RefPtr<WebCore::NodeList> list = node->document()->getElementsByTagName("label");
         unsigned length = list->length();
         for (unsigned i = 0; i < length; i++) {