[vm/ffi] Add missing free() to structs_test.dart
[vm/ffi] Add missing free() to structs_test.dart

Fixes crash on ASAN bot.

Change-Id: I70d0cc863b86c40d5c5c15841ecbbf2a91493e6c
Cq-Include-Trybots: luci.dart.try:vm-kernel-asan-linux-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108273
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Samir Jindel <5720a19f3a84f5405fa0776737a4149175e41e6a@google.com>

dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk