realtime: avoid needless signalling problems related to interference

realtime: avoid needless signalling problems related to interference

When we are considering an interfering segment, we want to first see
whether the segment has anything in it and only if so consider its
position.  That avoids is calling it a problem that the interfering
segment's position is not known (eg, because it has never been set).

To support this, split segment_interferes into two functions:
  segment_interferer: simply returns the possibly-interfering segment
      so that we can check whether it's relevant
  segment_interferer_does: does the hard work, dealing appropriately
      with errors

At all but the one call site of interest we can assert that the
getmovpos callback never fails, so we provide a helper function
  segment_interferes_simple
which does what segment_interferes used to (except that getmovpos
returning an error now causes an abort).