improve get-ip for getting failed

namesilo_ddns.sh

@@ -52,7 +52,7 @@ fi
 declare -g IP_ADDR_V4 IP_ADDR_V6 INV_HOSTS RECORDS FUNC_RETURN
 declare -g P_IPV4 P_IPV6 P_FORCE_UPDATE P_FORCE_FETCH
 declare -g PROJECT COPYRIGHT LICENSE HELP
-PROJECT="Namesilo DDNS without dependences v2.3 (2020.11.20)"
+PROJECT="Namesilo DDNS without dependences v2.4 (2020.11.29)"
 COPYRIGHT="Copyright (c) 2020 Mr.Jos"
 LICENSE="MIT License: <https://opensource.org/licenses/MIT>"
 HELP="Usage: namesilo_ddns.sh <command> ... [parameters ...]
@@ -285,16 +285,15 @@ function get_ip()
         done
     fi
 
-    if [[ $ARG == "-4" ]]; then
-        if [[ ${RES:=NULL} != "NULL" && $RES != ${IP_ADDR_V4:-NULL} ]]; then
-            echo "IPv4 address changed to {$RES}" >> $LOG
-        fi
-        IP_ADDR_V4=$RES
-    else
-        if [[ ${RES:=NULL} != "NULL" && $RES != ${IP_ADDR_V6:-NULL} ]]; then
-            echo "IPv6 address changed to {$RES}" >> $LOG
-        fi
-        IP_ADDR_V6=$RES
+    if [[ ${RES:=NULL} == "NULL" ]]; then
+        ## if ip-getting failed, cached ip will be applicated
+        return
+    elif [[ $ARG == "-4" && $RES != ${IP_ADDR_V4:-NULL} ]]; then
+        echo "IPv4 address changed to {$RES}" >> $LOG
+        IP_ADDR_V4="$RES"
+    elif [[ $ARG == "-6" && $RES != ${IP_ADDR_V6:-NULL} ]]; then
+        echo "IPv6 address changed to {$RES}" >> $LOG
+        IP_ADDR_V6="$RES"
     fi
 }