Update new usage of Pointer.NULL

Author: eregon

src/main/java/org/truffleruby/cext/CExtNodes.java

@@ -2028,7 +2028,7 @@ public abstract static class DataHolderCreate extends PrimitiveArrayArgumentsNod
 
         @Specialization
         protected DataHolder create(Object address) {
-            return new DataHolder(address, Pointer.NULL);
+            return new DataHolder(address, Pointer.getNullPointer(getContext()));
         }
     }
 

src/main/java/org/truffleruby/cext/DataHolder.java

@@ -26,17 +26,17 @@ public final class DataHolder implements TruffleObject {
     private Object pointer;
     private Object marker;
 
-    public DataHolder(Object address, Object marker) {
-        this.pointer = address;
+    public DataHolder(Object pointer, Object marker) {
+        this.pointer = pointer;
         this.marker = marker;
     }
 
     public Object getPointer() {
         return pointer;
     }
 
-    public void setPointer(Object address) {
-        this.pointer = address;
+    public void setPointer(Object pointer) {
+        this.pointer = pointer;
     }
 
     public Object getMarker() {