Memory leak in btr_defragment_calc_n_recs_for_size()

The function btr_defragment_calc_n_recs_for_size() that was added to MariaDB 10.1.1 as part of MDEV-5834 is leaking memory when a table contains enough many columns.

There is a call to rec_get_offsets() that may invoke mem_heap_create(), but there is no call to mem_heap_free(heap).

This should be reproducible with:

SET GLOBAL innodb_defragment = 1;

OPTIMIZE TABLE table_with_many_columns;

Simple fix: Add

if (UNIV_LIKELY_NULL(heap)) mem_heap_free(heap);

Better fix: Remove this kind of use of rec_get_offsets(), and implement a record field iterator that can be used for computing the size of a record. There is no need to compute any offsets except when some algorithm requires random access to the fields of an index record.