fix memory leak (#4126)

src/idl_parser.cpp

@@ -1836,6 +1836,7 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
   source_ = cursor_ = source;
   line_ = 1;
   error_.clear();
+  field_stack_.clear();
   builder_.Clear();
   // Start with a blank namespace just in case this file doesn't have one.
   namespaces_.push_back(new Namespace());