btr_page_split_and_insert(): Avoid dereferencing pointers to garbage on
the old page.

btr_cur_pessimistic_insert(): Pass pointer to big_rec_vec to
btr_cur_optimistic_insert().

trx_undo_prev_version_build(): Only invoke rec_set_field_extern_bits()
if n_ext_vect > 0.

row_ins_index_entry_low(): Simplify a debug assertion.

page_copy_rec_list_end_no_locks(): Make the loop slightly more readable.

page_delete_rec_list_end(): Delete records on compressed pages one by one.