[coap] avoid div-by-0 error in validating coap params (#4670)
[coap] avoid div-by-0 error in validating coap params (#4670)


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