fix [2319255] crash in openGL : from the media recorder stress test.

never call eglCreateImageKHR() with a NULL native buffer, which can
happen in OOM conditions.

libs/surfaceflinger/Layer.cpp

@@ -134,7 +134,14 @@ void Layer::reloadTexture(const Region& dirty)
 {
     Mutex::Autolock _l(mLock);
     sp<GraphicBuffer> buffer(getFrontBufferLocked());
-    int index = mFrontBufferIndex;
+    if (buffer == NULL) {
+        // this situation can happen if we ran out of memory for instance.
+        // not much we can do. continue to use whatever texture was bound
+        // to this context.
+        return;
+    }
+
+    const int index = mFrontBufferIndex;
 
     // create the new texture name if needed
     if (UNLIKELY(mTextures[index].name == -1U)) {