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.

scripts/mysqld_safe.sh

@@ -586,6 +586,7 @@ then
   then
     # User explicitly asked for syslog, so warn that it isn't used
     log_error "Can't log to error log and syslog at the same time.  Remove all --log-error configuration options for --syslog to take effect."
+    want_syslog=0
   fi
 
   # Log to err_log file