Use my_thread_var::stack_ends_here inside lf_pinbox_real_free() for address
where thread stack ends.

Remove LF_PINS::stack_ends_here.
It is not safe to assume that mysys_var that was used during pin allocation,
remains correct during free. E.g with binlog group commit in Innodb,
that frees pins for multiple Innodb transactions, it does not work
correctly.