[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/EventThread.cpp b/services/surfaceflinger/EventThread.cpp
index a1a0420..5c0e3b3 100644
--- a/services/surfaceflinger/EventThread.cpp
+++ b/services/surfaceflinger/EventThread.cpp
@@ -216,7 +216,7 @@
         size_t count = mDisplayEventConnections.size();
         for (size_t i=0 ; i<count ; ) {
             sp<Connection> connection(mDisplayEventConnections[i].promote());
-            if (connection != NULL) {
+            if (connection != nullptr) {
                 bool added = false;
                 if (connection->count >= 0) {
                     // we need vsync events because at least
@@ -346,7 +346,7 @@
         sp<Connection> connection =
                 mDisplayEventConnections.itemAt(i).promote();
         result.appendFormat("    %p: count=%d\n",
-                connection.get(), connection!=NULL ? connection->count : 0);
+                connection.get(), connection != nullptr ? connection->count : 0);
     }
 }