There can be cases when the optimizer calls ha_partition::records_in_range
when there are no matching partitions. So the DBUG_ASSERT of
!tot_used_partitions does assert.

Fixed by returning 0 instead when no matching partitions are found.

This will avoid the crash. records_in_range will then try to find the
biggest used partition, which will not find any partition and
records_in_range will then return 0, meaning non rows can be found.

Patch contributed by Davi Arnaut at twitter.