[router-table] exit earlier when `GetRouter()` for invalid router id (#4485)
[router-table] exit earlier when `GetRouter()` for invalid router id (#4485)

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