The UPDATE_CONTEXT_POINTERS actually breaks lots of debuggers tests. The root probl...

 The UPDATE_CONTEXT_POINTERS actually breaks lots of debuggers tests.  The root problem is that when arm unwind the stack, we will call this function
     hr = RtlpUnwindFunctionFull(pContext->Pc - (ULONG)ImageBase,
                                         (ULONG)ImageBase,
                                         &Rfe,
                                         pContext,
                                         &DummyEstablisherFrame,
                                         &DummyHandlerRoutine,
                                         &DummyHandlerData,
                                         NULL);  <-- UnwindParams

     This will set UnwindParams as NULL, and eventually passed to UPDATE_CONTEXT_POINTERS and UPDATE_FP_CONTEXT_POINTERS in RtlpPopRegisterMask. This will generate the AV.
     The fix is just simply checking the whether the Params is NULL or not.

[tfs-changeset: 1520758]