Coverity: circpadding: always check circpad_machine_current_state()

We usually check circpad_machine_current_state()'s return value, but in 2 places we don't. Coverity thinks that we're messing up in those cases.

This is CID 1447297 [circpad_estimate_circ_rtt_on_received] and 1447291 [circpad_is_token_removal_supported].

I suggest that we change our code to test the pointer in those cases. If we're sure that it can't be NULL, we can is an if (BUG(...)) check.