[vm] Fix leak of forwarding tables when isolate message sending fails.
[vm] Fix leak of forwarding tables when isolate message sending fails.

TEST=ci
Change-Id: I644834dcb49f197390dd71468b42b74c68a3cdb7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/255180
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@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