Initialized usable_keys from table->keys_in_use instead of ~0
in test_if_skip_sort_order(). It was possible that a disabled
index was used for sorting.


mysql-test/r/myisam.result:
  Bug#14616 - Freshly imported table returns error 124 when using LIMIT
  The test result.
mysql-test/t/myisam.test:
  Bug#14616 - Freshly imported table returns error 124 when using LIMIT
  The test case.