Don't write depth to JSON_Generator_State in some cases

For `JSON.generate` and `JSON::State#generate_new`, don't copy
generate_json_data::depth to JSON_Generator_State::depth.

In `JSON.generate`, the JSON_Generator_State is on the stack and
discarded anyway. In `JSON::State#generate_new`, we copy the struct to
avoid mutating the original one.

Author: etiennebarrie

ext/json/ext/generator/generator.c

@@ -1476,7 +1476,8 @@ static VALUE generate_json_try(VALUE d)
     return fbuffer_finalize(data->buffer);
 }
 
-static VALUE generate_json_ensure(VALUE d)
+// Preserves the deprecated behavior of State#depth being set.
+static VALUE generate_json_ensure_deprecated(VALUE d)
 {
     struct generate_json_data *data = (struct generate_json_data *)d;
     fbuffer_free(data->buffer);
@@ -1485,6 +1486,14 @@ static VALUE generate_json_ensure(VALUE d)
     return Qundef;
 }
 
+static VALUE generate_json_ensure(VALUE d)
+{
+    struct generate_json_data *data = (struct generate_json_data *)d;
+    fbuffer_free(data->buffer);
+
+    return Qundef;
+}
+
 static VALUE cState_partial_generate(VALUE self, VALUE obj, generator_func func, VALUE io)
 {
     GET_STATE(self);
@@ -1503,7 +1512,7 @@ static VALUE cState_partial_generate(VALUE self, VALUE obj, generator_func func,
         .obj = obj,
         .func = func
     };
-    return rb_ensure(generate_json_try, (VALUE)&data, generate_json_ensure, (VALUE)&data);
+    return rb_ensure(generate_json_try, (VALUE)&data, generate_json_ensure_deprecated, (VALUE)&data);
 }
 
 /* call-seq: