SR: Use BUG() instead of tor_assert() when we can

Example:

tor_assert(sr_state_get_phase() == SR_PHASE_REVEAL);

Should be replaced by:

if (BUG(sr_state_get_phase() != SR_PHASE_REVEAL))
  return;

The idea is to not kill the dirauth if this happens but still scream very loudly. Few other places in the SR subsystem can be found.

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