Opening certain tables that have different definitions in .MYI and
.frm may result in a server crash.

Compare .MYI and .frm definition when myisam table is opened. In case
definitions are diffirent refuse to open such table.

No test case, since it requires broken table.


storage/myisam/ha_myisam.cc:
  Compare .MYI and .frm definition when myisam table is opened. In case
  definitions are diffirent refuse to open such table.