Fix offline-snapshot analysis run-time crash

When expanding into a Stack Frame of a GC Root, ghc-debug-brick crashed with messages such as:

ghc-debug-brick: Cache Miss:RequestSRT 0x7444c59c2f80
CallStack (from HasCallStack):
  error, called at src/GHC/Debug/Client/Monad/Simple.hs:177:20 in ghc-debug-client-0.5.0.0-inplace:GHC.Debug.Client.Monad.Simple

In a pairing session with Matt, we came up with this fix.