INITIALIZE FOR RANDOM READ

When doing index scans and then swithing to
random scans, without restart, ha_rnd_init
is not called, when using the HANDLER SQL
interface.

Fixed by ensuring that when switching between
index and random scans there will always be a call to
ha_index/rnd_end and then ha_index/rnd_init.

Also checking return values from ha_index/rnd_init/end.

(updated with DBUG_ASSERT from reviewers suggestion).