Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsScriptC_LibGL.cpp b/rsScriptC_LibGL.cpp
index dbf2336..5c55c6b 100644
--- a/rsScriptC_LibGL.cpp
+++ b/rsScriptC_LibGL.cpp
@@ -103,12 +103,12 @@
 }
 
 void rsrClearFrameBufferObjectColorTarget(Context *rsc, uint32_t slot) {
-    rsc->mFBOCache.bindColorTarget(rsc, NULL, slot);
+    rsc->mFBOCache.bindColorTarget(rsc, nullptr, slot);
     rsc->mStateVertex.updateSize(rsc);
 }
 
 void rsrClearFrameBufferObjectDepthTarget(Context *rsc) {
-    rsc->mFBOCache.bindDepthTarget(rsc, NULL);
+    rsc->mFBOCache.bindDepthTarget(rsc, nullptr);
     rsc->mStateVertex.updateSize(rsc);
 }