fix memory leak (#4126)

diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index ef6e1f5..81e7fe2 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -1836,6 +1836,7 @@
   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());