MDEV-14256 MariaDB 10.2.10 can't SST with xtrabackup-v2

another followup for 4c2c057d.

there are six possible cases:
--port can be set or not.
--address can be set, not set, or set but without a port number

The correct behavior is:
1 both --port and --address have a port number
  - use it if it's the same, otherwise an error
2 only --port has the number (--address isn't set)
  - use the value from --port
3 only --port has the number (--address is set, but has no port)
  - use the value from --port
4 --port is unset, --address has the port number
  - use the value from --address
5 --port is unset, --address has no port number
  - use the value from --address, that is, port is empty string
6 --port is unset, --address is unset
  - port is unset (an error somewhere later)

case 5 wasn't handled correctly

scripts/wsrep_sst_common.sh

@@ -124,9 +124,9 @@ readonly WSREP_SST_OPT_BYPASS
 readonly WSREP_SST_OPT_BINLOG
 readonly WSREP_SST_OPT_CONF_SUFFIX
 
-if [ -n "${WSREP_SST_OPT_ADDR_PORT:-}" ]; then
+if [ -n "${WSREP_SST_OPT_ADDR:-}" ]; then
   if [ -n "${WSREP_SST_OPT_PORT:-}" ]; then
-    if [ "$WSREP_SST_OPT_PORT" != "$WSREP_SST_OPT_ADDR_PORT" ]; then
+    if [ -n "$WSREP_SST_OPT_ADDR_PORT" -a "$WSREP_SST_OPT_PORT" != "$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
     fi