Fix off by one error in advance

c++/src/capnp/compat/json-test.c++

@@ -196,6 +196,15 @@ KJ_TEST("basic json decoding") {
     KJ_EXPECT(root.getNull() == VOID);
   }
 
+  {
+    MallocMessageBuilder message;
+    auto root = message.initRoot<JsonValue>();
+    json.decodeRaw(kj::str("null"), root);
+
+    KJ_EXPECT(root.which() == JsonValue::NULL_);
+    KJ_EXPECT(root.getNull() == VOID);
+  }
+
   {
     MallocMessageBuilder message;
     auto root = message.initRoot<JsonValue>();

c++/src/capnp/compat/json.c++

@@ -488,7 +488,7 @@ public:
   }
 
   void advance(size_t numBytes = 1) {
-    KJ_REQUIRE(numBytes < remaining_.size(), "JSON message ends prematurely.");
+    KJ_REQUIRE(numBytes <= remaining_.size(), "JSON message ends prematurely.");
     remaining_ = kj::arrayPtr(remaining_.begin() + numBytes, remaining_.end());
   }