bug in long programs without data references

A program of 128 or more instructions e.g. `MOV R0, #0` that has no defined data (`DCD`, `DCB` etc) will give parse failure from line 425 of `ExecutionTop.fs`.

### To reproduce

NB this bug is present on Visual2 master branch and (identically) hlp2019-test.

- Start Visual2
- load or enter a file consisting of 128 (or more) `MOV R0, #0`
- Click `Run`

### expected result

Program runs taking 128 steps and ends up `Finished`

### Actual Result

Nothing happens in GUI. If you open dev tools the failwithf message is shown in the window

Start at line 425 of ExecutionTop.fs and reproduce the failure in Visual2 with a file containing 128 or more `MOV R0, #0` instructions and no data definitions such as DCD or DCB. Compare the behavior with the expected 128 execution steps and a final `Finished` state, using the dev tools failwithf message as the failure signal.

Written by the indexing model from the issue text.