fix get_ipv4_addr(@interface) usage

get_ipv4_addr(@interface) returns a string not list, so get_ipv4_addr(@interface)[0] only got the first character of IP, which raises an error.

Author: 朱雄宇

modules/auxiliary/spoof/arp/arp_poisoning.rb

@@ -79,7 +79,7 @@ def run
       raise RuntimeError ,'Source MAC is not in correct format' unless is_mac?(@smac)
 
       @sip = datastore['LOCALSIP']
-      @sip ||= get_ipv4_addr(@interface)[0] if @netifaces
+      @sip ||= get_ipv4_addr(@interface) if @netifaces
       raise "LOCALSIP is not defined and can not be guessed" unless @sip
       raise "LOCALSIP is not an ipv4 address" unless Rex::Socket.is_ipv4?(@sip)