hwc: Fix to avoid heap corruption

- Check for MAX_NUM_APP_LAYERS before updating yuv indices array.
- Fall back to GPU composition when number of app layers exceeds
  MAX_NUM_APP_LAYERS to avoid heap corruption.

Change-Id: Ieb91b705a0a5f50ce2f8829d1f1ee048d44b7d2e
diff --git a/libhwcomposer/hwc_copybit.cpp b/libhwcomposer/hwc_copybit.cpp
index 1f724de..2adb2ab 100644
--- a/libhwcomposer/hwc_copybit.cpp
+++ b/libhwcomposer/hwc_copybit.cpp
@@ -153,7 +153,7 @@
         return false;
     }
 
-    if (ctx->listStats[dpy].numAppLayers > MAX_NUM_LAYERS) {
+    if (ctx->listStats[dpy].numAppLayers > MAX_NUM_APP_LAYERS) {
         // Reached max layers supported by HWC.
         return false;
     }