[coap] fix write outside buffer bounds (#325)
[coap] fix write outside buffer bounds (#325)

Lack of (unsigned char *) cast a pointer to structure pdu->hdr is
causing writes outside CoAP PDU buffer bounds.
bukepo/borderrouter,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,bukepo/borderrouter,jwhui/borderrouter,bukepo/borderrouter,openthread/borderrouter,bukepo/borderrouter,jwhui/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,openthread/borderrouter,openthread/borderrouter