Reject invalid non-null tokens beginning with n (#93)

* Add test cases for invalid n tokens

Tokens that begin with `n` should not be treated as `null`.

* Reject non-null tokens beginning with n

In 0f609dd, a regression was introduced
where the parser would read any token that started with n as null.

The `.type()` method returns `json_type::null` simply if the token
begins with `n`, and it does not check whether the token is actually
valid.

Adding an `is_null()` call here ensures this and returns an error if the
token starts with `n` but is not `null`. Calling `.value()` on the
returned value will raise it as an exception if `is_null()` returned an
error.

Currently with simdjson 3.10.1, there is a bug where calling `is_null`
on a document object lacks this behaviour, so currently this patch only
fixes the problem for non-scalar documents.

* Comment out broken tests for now

jsonexamples/invalid/nil_token.json

@@ -0,0 +1 @@
+[nil]

jsonexamples/invalid/nil_token_scalar.json

@@ -0,0 +1 @@
+nil

jsonexamples/invalid/nully_token.json

@@ -0,0 +1 @@
+[nully]

jsonexamples/invalid/nully_token_scalar.json

@@ -0,0 +1 @@
+nully

spec/compile_spec.lua

@@ -107,7 +107,11 @@ if tonumber(major) >= 5 and tonumber(minor) >= 3 then
 end
 
 local invalid_files = {
-    "bool_trailing.json"
+    "bool_trailing.json",
+    "nil_token.json",
+    -- "nil_token_scalar.json",
+    "nully_token.json",
+    -- "nully_token_scalar.json"
 }
 
 describe("Make sure invalid files are not accepted", function()

src/luasimdjson.cpp

@@ -121,7 +121,10 @@ void convert_ondemand_element_to_table(lua_State *L, T& element) {
       break;
 
     case ondemand::json_type::null:
-      lua_pushlightuserdata(L, NULL);
+      // calling is_null().value() will trigger an exception if the value is invalid
+      if (element.is_null().value()) {
+        lua_pushlightuserdata(L, NULL);
+      }
       break;
   }
 }