[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index cc675c4..63f6781 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -265,7 +265,7 @@
     if (!mCurrentCrop.isEmpty()) {
         // if the buffer crop is defined, we use that
         crop = mCurrentCrop;
-    } else if (getBE().compositionInfo.mBuffer != NULL) {
+    } else if (getBE().compositionInfo.mBuffer != nullptr) {
         // otherwise we use the whole buffer
         crop = getBE().compositionInfo.mBuffer->getBounds();
     } else {