Problem: inserting a record we don't set unused null bits in the
record buffer if no default field values used.
That may lead to wrong live checksum calculation.

Fix: set unused null bits in the record buffer in such cases.


mysql-test/r/myisam.result:
  Fix for bug#49465: valgrind warnings and incorrect live checksum...
    - test result.
mysql-test/t/myisam.test:
  Fix for bug#49465: valgrind warnings and incorrect live checksum...
    - test case.
sql/sql_insert.cc:
  Fix for bug#49465: valgrind warnings and incorrect live checksum...
    - set unused null bits to 1 in the record buffer in case we
  don't call restore_record() before a fill_record() call
  (when no default values used).