hwui: fix possible null pointer de-refrence

add layer check for  null before de-refrencing it.

issue link: http://code.google.com/p/android/issues/detail?id=61622

Change-Id: I54fc21ec594effcfb1eededd80dd5b0b52645c4c

libs/hwui/LayerRenderer.cpp

@@ -339,8 +339,10 @@ void LayerRenderer::destroyLayerDeferred(Layer* layer) {
 
 void LayerRenderer::flushLayer(Layer* layer) {
 #ifdef GL_EXT_discard_framebuffer
+    if (!layer) return;
+
     GLuint fbo = layer->getFbo();
-    if (layer && fbo) {
+    if (fbo) {
         // If possible, discard any enqueud operations on deferred
         // rendering architectures
         if (Extensions::getInstance().hasDiscardFramebuffer()) {