Bug #12546953 "SHOW VARIABLES LIKE 'DATADIR';" RETURN EMPTY.

Problem:
===========================================================
If mysqld daemon is started without a --datadir option
option, and we issue the SHOW VARIABLES LIKE 'DATADIR';SQL command
at the client it returns an empty path. This is because
mysql_real_data_home_ptr is being reset to NULL by Sys_var_charptr
constructor call when the datadir is not given either through
configuration file (no-defaults) or through mysqld parameters.

Solution:
===========================================================
mysql_real_data_home is an array which stores the path of the datadir
and mysql_real_data_home_ptr is the pointer to it. The pointer is
being set to NULL at the Sys_datadir, which is of type Sys_var_charptr,
constructor call. This is because at Sys_datadir call the def_val 
parameter was being passed with DEFAULT(0) which is now replaced with
DEFAULT(mysql_real_data_home). The patch has been tested manually as it
is not possible to start mtr without a default config file.

sql/sys_vars.cc

@@ -659,7 +659,7 @@ static Sys_var_ulong Sys_connect_timeout(
 static Sys_var_charptr Sys_datadir(
        "datadir", "Path to the database root directory",
        READ_ONLY GLOBAL_VAR(mysql_real_data_home_ptr),
-       CMD_LINE(REQUIRED_ARG, 'h'), IN_FS_CHARSET, DEFAULT(0));
+       CMD_LINE(REQUIRED_ARG, 'h'), IN_FS_CHARSET, DEFAULT(mysql_real_data_home));
 
 #ifndef DBUG_OFF
 static Sys_var_dbug Sys_dbug(