from next-mr-bugfixing:
BUG#54682 "set sql_select_limit=0 does not work"; let SQL_SELECT_LIMIT=0
work like it does in 5.1.


mysql-test/suite/sys_vars/r/sql_select_limit_func.result:
  before the fix, the SET would emit a warning (0 being rounded up to 1)
  and SELECTs would return one row.
sql/sys_vars.cc:
  0 is allowed, it means an implicit LIMIT 0 (i.e. no rows returned)