Add --pointer-check to goto-instrument --drop-unused-functions call

[celinval]

As pointed out by @tautschnig here (diffblue/cbmc#6983 (comment)), we should add --pointer-check while droping unused functions to ensure that CBMC fails the verification if it cannot find a suitable pointer for a function call.

[celinval]

@tautschnig Just double checking, this is no longer needed, right?

That said, do you still recommend enabling all checks before invoking goto-instrument --drop-unused-functions?

[tautschnig]

My apologies for the delay: yes, this should no longer be necessary. Also, I don't immediately see a reason as to why you'd need to enable checks before --drop-unused-functions.

[celinval]

No worries! Thank you.