Fix #700 null pointer exception (#702)

Author: apinprastya

httplib.h

@@ -2949,7 +2949,7 @@ class MultipartFormDataParser {
   bool is_valid() const { return is_valid_; }
 
   template <typename T, typename U>
-  bool parse(const char *buf, size_t n, T content_callback, U header_callback) {
+  bool parse(const char *buf, size_t n, const T &content_callback, const U &header_callback) {
 
     static const std::regex re_content_disposition(
         "^Content-Disposition:\\s*form-data;\\s*name=\"(.*?)\"(?:;\\s*filename="
@@ -4273,8 +4273,8 @@ inline bool Server::read_content_core(Stream &strm, Request &req, Response &res,
       }
       return true;
       */
-      return multipart_form_data_parser.parse(buf, n, std::move(multipart_receiver),
-                                              std::move(mulitpart_header));
+      return multipart_form_data_parser.parse(buf, n, multipart_receiver,
+                                              mulitpart_header);
     };
   } else {
     out = std::move(receiver);