BUG#12929941: SEGFAULT IN STRMAKE/CONVERT_DIRNAME WHEN USING --RELAY-LOG=SLAVE-RELAY-BIN
      
Problem Description: 
      The global variable 'char *mysql_real_data_home_ptr'
      is declared and initialized to the address of the
      global buffer char mysql_real_data_home[] in mysqld.cc.
      Then, the constructor for Sys_datadir overwrites
      mysql_real_data_home_ptr with NULL. In the case when value
      datadir is not given in both command-line as well as in
      cnf file and the default path (<basedir>/data) needs to be
      used, the mysql_real_data_home_ptr is not made to point to
      mysql_real_data_home and still has NULL value. 
      After WL#5465, we use mysql_real_data_home_ptr when it still
      holds NULL as no cli or cnf file arguments were given. In
      particular, rpl_make_log_name passes mysql_real_data_home_ptr
      to fn_format. This caused a coredump because fn_format did not
      expect NULL.
      The following combinations caused the core dump
      1.unspecified but existing empty default data dir and
        --log-bin 
      2.unspecified but existing empty default data dir and
        --relay-log=<some name for relay log>
      
             
Solution: 
      We solve this problem by making mysql_real_data_home_ptr to
      point to the mysql_real_data_home if it is null before passing
      it to fn_format function because by this time we know that
      mysql_real_data_home is correctly initialized and holds the 
      final value of datadir path. We have also added asserts in
      fn_format() to check if the dir is null.