[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/Client.cpp b/services/surfaceflinger/Client.cpp
index ea6541a..a69940a 100644
--- a/services/surfaceflinger/Client.cpp
+++ b/services/surfaceflinger/Client.cpp
@@ -206,7 +206,7 @@
 
 status_t Client::clearLayerFrameStats(const sp<IBinder>& handle) const {
     sp<Layer> layer = getLayerUser(handle);
-    if (layer == NULL) {
+    if (layer == nullptr) {
         return NAME_NOT_FOUND;
     }
     layer->clearFrameStats();
@@ -215,7 +215,7 @@
 
 status_t Client::getLayerFrameStats(const sp<IBinder>& handle, FrameStats* outStats) const {
     sp<Layer> layer = getLayerUser(handle);
-    if (layer == NULL) {
+    if (layer == nullptr) {
         return NAME_NOT_FOUND;
     }
     layer->getFrameStats(outStats);