Only add --defaults-extra-file=$DATADIR/my.cnf if $DATADIR/my.cnf is readable.


scripts/mysqld_safe.sh:
  Only add --defaults-extra-file=$DATADIR/my.cnf if $DATADIR/my.cnf is readable.
  Without this patch, running mysqld_safe gives:
  
  Could not open required defaults file: /opt/home/tim/m/50/m/data/my.cnf
  Fatal error in defaults handling. Program aborted
  Could not open required defaults file: /opt/home/tim/m/50/m/data/my.cnf
  Fatal error in defaults handling. Program aborted
  
  These errors are printed by my_print_defaults; mysqld also encounters the
  same problem and quits.