Add null pointer check to avoid system crash.

Add null pointer check to avoid system crash.
Meet the below crash log during some app operations then app exits.

F DEBUG   : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x78
F DEBUG   : Cause: null pointer dereference
F DEBUG   : backtrace:
F DEBUG   :       #00 pc 0000000000323858  /system/lib64/libhwui.so
(android::uirenderer::DeferredLayerUpdater::apply()+40)
F DEBUG   :       #1 pc 0000000000323040  /system/lib64/libhwui.so
(_ZNSt3__110__function6__funcIZN7android10uirenderer12renderthread13
DrawFrameTask11postAndWaitEvE3$_0NS_9allocatorIS6_EEFvvEEclEv$c1671e
787f244890c877724752face20+416)

Bug: b/243310920
Signed-off-by: Jessie Hao <juan.hao@nxp.com>
Change-Id: I77226be2bd235852675bb4f7a29589e360269b33

libs/hwui/renderthread/DrawFrameTask.cpp

@@ -243,7 +243,9 @@ bool DrawFrameTask::syncFrameState(TreeInfo& info) {
     mContext->unpinImages();
 
     for (size_t i = 0; i < mLayers.size(); i++) {
-        mLayers[i]->apply();
+        if (mLayers[i]) {
+            mLayers[i]->apply();
+        }
     }
     mLayers.clear();
     mContext->setContentDrawBounds(mContentDrawBounds);