MDEV-6220 mysqldump will not backup database with --flush-logs parameter and log_error my.cnf parameter defined

some checks were "if [ -n "$err_log" ]", others were "if [ $want_syslog -eq 0 ]",
so when both are set, error log file was only partially initialized.
To avoid this ambiguity we reset want_syslog when error log file is used.