Fix out-of-bounds vector access in StructDef::Deserialize (#8988)

* Fix out-of-bounds vector access in StructDef::Deserialize

* Fix syntax: use error_ instead of error()

src/idl_parser.cpp

diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index fe2878a..b1bdffa 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -4132,7 +4132,15 @@
   sortbysize = attributes.Lookup("original_order") == nullptr && !fixed;
   const auto& of = *(object->fields());
   auto indexes = std::vector<uoffset_t>(of.size());
-  for (uoffset_t i = 0; i < of.size(); i++) indexes[of.Get(i)->id()] = i;
+  for (uoffset_t i = 0; i < of.size(); i++) {
+  uint16_t field_id = of.Get(i)->id();
+  if (field_id >= of.size()) {
+    parser.error_ = "Field ID " + std::to_string(field_id) + 
+                    " exceeds field count " + std::to_string(of.size());
+    return false;
+  }
+  indexes[field_id] = i;
+}
   size_t tmp_struct_size = 0;
   for (size_t i = 0; i < indexes.size(); i++) {
     auto field = of.Get(indexes[i]);