MDEV-20627 : Galera 4 not able to report proper wsrep_incoming_addresses

wsrep_server_incoming_address function always returned value of the
wsrep_node_incoming_address even when actual incoming address
was resolved to inc_addr variable. Fixed by returning inc_addr
if it does contain incoming address.

sql/wsrep_mysqld.cc

@@ -660,7 +660,11 @@ static std::string wsrep_server_incoming_address()
   }
 
  done:
+  if (!strlen(inc_addr))
     ret= wsrep_node_incoming_address;
+  else
+    ret= inc_addr;
+  WSREP_DEBUG("wsrep_incoming_address = %s", ret.c_str());
   return ret;
 }