[tests/ffi]: Fix failing pointer arithmetic test.

Closes #54397.

R=dacoharkes@google.com

Cq-Include-Trybots: luci.dart.try:vm-aot-asan-linux-release-x64-try,vm-asan-linux-release-x64-try
Change-Id: I4ae2cbc7f4eee1bfcd6d608dbc0d838da703c7e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/342460
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Derek Xu <derekx@google.com>
Auto-Submit: Shikhar <shikharish05@gmail.com>

tests/ffi/pointer_arithmetic_operators_test.dart

@@ -9,6 +9,7 @@ import 'package:ffi/ffi.dart';
 
 void main() {
   Pointer<Int32> p = calloc<Int32>(3);
+  final p1 = p;
   p.value = 1;
   (p + 1).value = 2;
   (p + 2).value = 3;
@@ -17,4 +18,5 @@ void main() {
   Expect.equals(3, p.value);
   p -= 1;
   Expect.equals(2, p.value);
+  calloc.free(p1);
 }