sql/mysqld.cc:4564:

    if (opt_bin_logname && 

        opt_bin_logname[strlen(opt_bin_logname) - 1] == FN_LIBCHAR)

But when I pass --log-bin without filename the opt_bin_logname is actually an empty string and then the statement accesses invalid memory address. These same code lines work fine on MySQL 5.1, so apparently at some point opt_bin_logname was changed to have empty string instead of NULL.