[lldb] Correctly check and report error states in StackFrame.cpp (#74414)

This commits fixes a few subtle bugs where the method:
1. Declares a local `Status error` which eclipses the method's parameter
`Status &error`.
- The method then sets the error state to the local `error` and returns
without ever touching the parameter `&error`.
- This effectively traps the error state and its message from ever
reaching the caller.
- I also threw in a null pointer check in case the callee doesn't set
its `Status` parameter but returns `0`/`nullptr`.

2. Declares a local `Status deref_error` (good), passes it to the
`Dereference` method (also good), but then checks the status of the
method's `Status &error` parameter (not good).
- The fix checks `deref_error` instead and also checks for a `nullptr`
return value.
- There's a good opportunity here for a future PR that changes the
`Dereference` method to fold an error state into the `ValueObject`
return value's `m_error` instead of using a parameter.
...