Problem was that ha_partition::records_in_range called
records_in_range for all non pruned partitions, even if
an estimate should be given.

Solution is to only use 1/3 of the partitions (up to 10) for
records_in_range and estimate the total from this subset.
(And continue until a non zero return value from the called
partitions records_in_range is given, since 0 means no rows
will match.)