[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/RenderEngine/Program.cpp b/services/surfaceflinger/RenderEngine/Program.cpp
index baf92eb..225bcf0 100644
--- a/services/surfaceflinger/RenderEngine/Program.cpp
+++ b/services/surfaceflinger/RenderEngine/Program.cpp
@@ -115,7 +115,7 @@
     GLint l;
     glGetShaderiv(shader, GL_SHADER_SOURCE_LENGTH, &l);
     char* src = new char[l];
-    glGetShaderSource(shader, l, NULL, src);
+    glGetShaderSource(shader, l, nullptr, src);
     result.append(src);
     delete[] src;
     return result;