[dhcp6-client] only close socket if no agents exist (#5169)
[dhcp6-client] only close socket if no agents exist (#5169)

lanyuwen/openthread,lanyuwen/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,chshu/openthread,openthread/openthread,bukepo/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,abtink/openthread,chshu/openthread,abtink/openthread