Crash may happen when selecting from a merge table that has underlying
tables with less indexes than in a merge table itself.

If number of keys in merge table is not bigger than requested key number,
return error.


myisammrg/myrg_open.c:
  Store min(number of keys) in m_info instead of number of keys in last
  underlying table.
myisammrg/myrg_queue.c:
  Return error if inx passed to _myrg_init_queue function is not less
  than number of keys.
mysql-test/r/merge.result:
  A test case for bug#21617.
mysql-test/t/merge.test:
  A test case for bug#21617.
mysys/queues.c:
  Replaced annoying ifndef DBUG_OFF with DBUG_ASSERT, fixed coding style.
  The problem was that having queue overrun in debug build was hidden
  with this ifdef.