MDEV-13968 sst fails with "WSREP_SST_OPT_PORT: readonly variable"

followup for 990c4318a779645ee

fix broken script (set -u)

scripts/wsrep_sst_common.sh

@@ -129,8 +129,8 @@ done
 readonly WSREP_SST_OPT_BYPASS
 readonly WSREP_SST_OPT_BINLOG
 
-if [ -n "$WSREP_SST_OPT_ADDR_PORT" ]; then
-  if [ -n "$WSREP_SST_OPT_PORT" ]; then
+if [ -n "${WSREP_SST_OPT_ADDR_PORT:-}" ]; then
+  if [ -n "${WSREP_SST_OPT_PORT:-}" ]; then
     if [ "$WSREP_SST_OPT_PORT" != "$WSREP_SST_OPT_ADDR_PORT" ]; then
       wsrep_log_error "port in --port=$WSREP_SST_OPT_PORT differs from port in --address=$WSREP_SST_OPT_ADDR"
       exit 2