[cli] avoid NULL dereference with `udp send -s 0` command (#4164)
[cli] avoid NULL dereference with `udp send -s 0` command (#4164)


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