Make WebPluginContainerImpl::invalidateRect check for NULL renderer

The plugin might attempt to call invalidateRect while it is being
destroyed, in which case its renderer could be NULL.

BUG=432045

Review URL: https://codereview.chromium.org/723733002

git-svn-id: svn://svn.chromium.org/blink/trunk@185308 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

@@ -137,6 +137,8 @@ void WebPluginContainerImpl::invalidateRect(const IntRect& rect)
         return;
 
     RenderBox* renderer = toRenderBox(m_element->renderer());
+    if (!renderer)
+        return;
 
     IntRect dirtyRect = rect;
     dirtyRect.move(renderer->borderLeft() + renderer->paddingLeft(),