The test mariabackup.incremental_backup revealed a memory leak
in have_queries_to_wait_for(). The problem is that
xb_mysql_query() is being invoked with bool use_result=true
but the result is not being freed by mysql_store_result().
There are similar leaks in other functions.

have_queries_to_wait_for(): Invoke mysql_free_result() to
clean up after the mysql_store_result() that was invoked
by xb_mysql_query().

select_incremental_lsn_from_history(): Plug the leak on failure.

kill_long_queries(): Plug the memory leak.
(This function always leaked memory when it was called.)