flatcc requires even empty vectors are aligned

Asserts.c

#include <assert.h>
#include <stdio.h>

#include "nanoarrow/ipc/flatcc_generated.h"

static const char dump[] = {
    0x14, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0a, 0x00,
    0x0e, 0x00, 0x06, 0x00, 0x05, 0x00, 0x08, 0x00, 0x0a, 0x00, 0x00, 0x00,
    0x00, 0x03, 0x04, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0a, 0x00,
    0x0c, 0x00, 0x00, 0x00, 0x04, 0x00, 0x08, 0x00, 0x0a, 0x00, 0x00, 0x00,
    0x0c, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};

int main() {
#define ns(x) FLATBUFFERS_WRAP_NAMESPACE(org_apache_arrow_flatbuf, x)

  // assert(ns(Message_verify_as_root(dump, sizeof(dump))) == flatcc_verify_ok);
  ns(Message_table_t) message = ns(Message_as_root(dump));

  assert(ns(Message_header_type(message)) == ns(MessageHeader_RecordBatch));
  ns(RecordBatch_table_t) record_batch =
      (ns(RecordBatch_table_t))ns(Message_header_get(message));

  ns(Buffer_vec_t) buffers = ns(RecordBatch_buffers_get(record_batch));
  assert(ns(Buffer_vec_len(buffers)) == 0);

  static_assert(alignof(ns(Buffer_t)) == 8, "");
  int offset = (const char *)buffers - dump;
  // assert(offset % 8 == 0);
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.14)
project(flatcc_repro)

include(FetchContent)
include(ExternalProject)

# fetch flatcc v0.6.1
FetchContent_Declare(
  flatcc
  GIT_REPOSITORY https://github.com/dvidelabs/flatcc.git
  GIT_TAG v0.6.1
  PATCH_COMMAND
    # Minor patch to get include dirs right for the flatccrt target
    git reset --hard HEAD && git apply "${CMAKE_SOURCE_DIR}/flatcc.patch"
)
option(FLATCC_RTONLY "enable build of runtime library only" ON)
option(FLATCC_INSTALL "enable install targets" ON)
FetchContent_MakeAvailable(flatcc)

add_executable(repro Asserts.c)
target_link_libraries(repro PRIVATE flatccrt)

# fetch flatcc_generated.h
ExternalProject_Add(
  nanoarrow
  GIT_REPOSITORY https://github.com/apache/arrow-nanoarrow.git
  GIT_TAG 16f4306f58193eff18013cc75d8b7b49f1ad4108
  CONFIGURE_COMMAND ""
  BUILD_COMMAND ""
  INSTALL_COMMAND ""
)
ExternalProject_Get_property(nanoarrow SOURCE_DIR)
target_include_directories(repro PRIVATE "${SOURCE_DIR}/src")

flatcc.patch

diff --git a/src/runtime/CMakeLists.txt b/src/runtime/CMakeLists.txt
index 127e2a4..8f81f74 100644
--- a/src/runtime/CMakeLists.txt
+++ b/src/runtime/CMakeLists.txt
@@ -1,7 +1,3 @@
-include_directories (
-    "${PROJECT_SOURCE_DIR}/include"
-)
-
 add_library(flatccrt
     builder.c
     emitter.c
@@ -10,6 +6,10 @@ add_library(flatccrt
     json_parser.c
     json_printer.c
 )
+target_include_directories(
+    flatccrt
+    PUBLIC "${PROJECT_SOURCE_DIR}/include"
+)
 
 if (FLATCC_INSTALL)
     install(TARGETS flatccrt DESTINATION ${lib_dir})