Added a check, if the table, which we are going to create, is open.
This can happen if a MERGE mapped table is TRUNCATEd.


myisam/mi_open.c:
  Bug#8306 - TRUNCATE leads to index corruption
  Made test_if_reopen() globally available.
myisam/myisamdef.h:
  Bug#8306 - TRUNCATE leads to index corruption
  Declared test_if_reopen() as globally available.
mysql-test/r/myisam.result:
  Bug#8306 - TRUNCATE leads to index corruption
  The test result.
mysql-test/t/myisam.test:
  Bug#8306 - TRUNCATE leads to index corruption
  The test case.