[vm] Fix NPE in IsFfiCompound
[vm] Fix NPE in IsFfiCompound

Ran into this error when trying to set up a finalizer with an Object
owner. The super class is null in this case.

Change-Id: I27f67a738c72c0b433e367257c638a6d3d2e495f
TEST=ci and manual testing
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199602
Commit-Queue: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk