Assertion failure may happen with falcon + partition + select for
update. This may affect other engines as well.

Though assertion failure is fixed with this patch, falcon still
deadlocks when running falcon_bug_28026.test.


sql/ha_partition.cc:
  In case rnd_next fails, we must call ha_rnd_end. This is done
  conditionally (NO_CURRENT_PART_ID != m_part_spec.start_part)
  in ha_partition::ha_rnd_end. Thus we may not reset
  m_part_spec.start_part in case rnd is not ended.