Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsThreadIO.cpp b/rsThreadIO.cpp
index 8a0a5dc..87ecdf2 100644
--- a/rsThreadIO.cpp
+++ b/rsThreadIO.cpp
@@ -79,7 +79,7 @@
 
 void ThreadIO::coreSetReturn(const void *data, size_t dataLen) {
     uint32_t buf;
-    if (data == NULL) {
+    if (data == nullptr) {
         data = &buf;
         dataLen = sizeof(buf);
     }
@@ -89,7 +89,7 @@
 
 void ThreadIO::coreGetReturn(void *data, size_t dataLen) {
     uint32_t buf;
-    if (data == NULL) {
+    if (data == nullptr) {
         data = &buf;
         dataLen = sizeof(buf);
     }