tokener: clear pending surrogate on reset

Discard the previous document's high surrogate when resetting the tokener.
Otherwise the next Unicode escape can add a replacement character or
combine with a surrogate from the abandoned document.

Extend the existing incremental parse tests for incomplete input and a
fatal error, using independent next-document string expectations.

Fixes #984.
Reported-by: GeekQAQ (GitHub)
Assisted-by: OpenAI Codex (AI)
Reviewed-by: GPT-6 Luna (AI)

json_tokener.c

@@ -214,6 +214,7 @@ void json_tokener_reset(struct json_tokener *tok)
 		json_tokener_reset_level(tok, i);
 	tok->depth = 0;
 	tok->err = json_tokener_success;
+	tok->high_surrogate = 0;
 }
 
 struct json_object *json_tokener_parse(const char *str)