Update dmlc-core, fix default ctors of NodeEntry (#3017)

dmlc-core @ 3ffea869

-Subproject commit 2b5b1ba9c1103f438d164aca32da7cffd8cd48e8
+Subproject commit 3ffea8694adf9c0363f9abbf162dc0e4a45b22c5

nnvm/include/nnvm/node.h

@@ -56,8 +56,18 @@ struct NodeEntry {
     version(version)
   {}
 
+  explicit NodeEntry(NodePtr node):
+    node(std::move(node)),
+    index(),
+    version()
+  {}
+
+  /**
+   * MXNet assumes that a node with a null ptr doesn't have a gradient attached. Don't change this
+   * constructor.
+   */
   NodeEntry():
-    node(),
+    node(nullptr),
     index(),
     version()
   {}