added system command
myisammrg/myrg_open.c
    fixed coredump when opening an empty union table
sql/sql_repl.cc
    fixed warnings, remove confusing comment, fixed coredump in
    change master to when master.info was corrupted

added new replication test case


client/mysqltest.c:
  added system command
myisammrg/myrg_open.c:
  fixed coredump when opening an empty union table
sql/sql_repl.cc:
  fixed warnings, remove confusing comment, fixed coredump in
  change master to when master.info was corrupted