Fix behaviour of freeNatives()

Summary:
@public

Prevents repeated deallocation of weak references.

Reviewed By: pasqualeanatriello

Differential Revision: D9131551

fbshipit-source-id: bc79596e056ae0657a55146ad786422fd0f5badc

java/jni/YGJNI.cpp

@@ -384,7 +384,7 @@ jlong jni_YGNodeCloneNoProps(
   return jni_YGNodeClone(cls, nativePointer, clonedJavaObject, nullptr);
 }
 
-void jni_YGNodeFree(alias_ref<jobject> thiz, jlong nativePointer) {
+void jni_YGNodeFree(alias_ref<jclass> thiz, jlong nativePointer) {
   if (nativePointer == 0) {
     return;
   }