[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/LayerRejecter.cpp b/services/surfaceflinger/LayerRejecter.cpp
index e864607..a5f0b98 100644
--- a/services/surfaceflinger/LayerRejecter.cpp
+++ b/services/surfaceflinger/LayerRejecter.cpp
@@ -41,7 +41,7 @@
     mFreezeGeometryUpdates(freezePositionUpdates) {}
 
 bool LayerRejecter::reject(const sp<GraphicBuffer>& buf, const BufferItem& item) {
-    if (buf == NULL) {
+    if (buf == nullptr) {
         return false;
     }