Strengthen checkMemoryBoundInvariants check for Node0

I think this is probably checked elsewhere, but let's check here too for
consistency

ConflictSet.cpp

@@ -524,6 +524,8 @@ std::string getSearchPath(Node *n);
 // Induction hypothesis is that each node's surplus is >= kMinNodeSurplus
 constexpr int kBytesPerKey = 112;
 constexpr int kMinNodeSurplus = 80;
+// Cound the entry itself as a child
+constexpr int kMinChildrenNode0 = 1;
 constexpr int kMinChildrenNode3 = 2;
 constexpr int kMinChildrenNode16 = 4;
 constexpr int kMinChildrenNode48 = 17;
@@ -3886,7 +3888,7 @@ checkMaxVersion(Node *root, Node *node, InternalVersionT oldestVersion,
   int minNumChildren;
   switch (node->getType()) {
   case Type_Node0:
-    minNumChildren = 0;
+    minNumChildren = kMinChildrenNode0;
     break;
   case Type_Node3:
     minNumChildren = kMinChildrenNode3;