Problem: the server missed the fact that one can read from 
2 indexes alternately using HANDLER interface.

Fix: check if the same (initialized) index is involved
reading next/prev values from the index.


mysql-test/r/handler_myisam.result:
  Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
    - test result.
mysql-test/t/handler_myisam.test:
  Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
    - test case.
sql/sql_handler.cc:
  Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
    - check if we use the same (initialized) index 
  to read next/prev values from the index.