[VM] Fix nullptr check in DFE
[VM] Fix nullptr check in DFE

TEST=all tests running from file and none (to my knowledge) testing
the failure case as that's going to be a weird timing issue.

Change-Id: I997515587e86b324880fdd9fe11e86343e8bb973
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199423
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@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