Bug #12589928: MEMORY LEAK WHEN RUNNING SYS_VARS.SECURE_FILE_PRIV

This is the 5.1 version of the fix.
      
Need to free the memory allocated by the option parsing code for empty 
strings when resetting the pointer to NULL.
No test case needed, as the existing ones already cover this path.

sql/mysqld.cc

@@ -9063,6 +9063,7 @@ static int fix_paths(void)
   {
     if (*opt_secure_file_priv == 0)
     {
+      my_free(opt_secure_file_priv, MYF(0));
       opt_secure_file_priv= 0;
     }
     else