[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/DisplayHardware/FramebufferSurface.h b/services/surfaceflinger/DisplayHardware/FramebufferSurface.h
index 4186b7a..eaa5455 100644
--- a/services/surfaceflinger/DisplayHardware/FramebufferSurface.h
+++ b/services/surfaceflinger/DisplayHardware/FramebufferSurface.h
@@ -80,7 +80,7 @@
     // on/off.
     android_dataspace mDataSpace;
 
-    // mCurrentBuffer is the current buffer or NULL to indicate that there is
+    // mCurrentBuffer is the current buffer or nullptr to indicate that there is
     // no current buffer.
     sp<GraphicBuffer> mCurrentBuffer;