bugfix: #17 coap_opt_size underestimates space usage
bugfix: #17 coap_opt_size underestimates space usage

Fixed calculation of option size in coap_new_error_response(). As
only some options are copied from the request, the delta value can
increase and thus require more space to encode. This fix closes
https://sourceforge.net/p/libcoap/bugs/17/ (thanks to chrysn and
tbahls for pointing this out).

RIOT-OS/libcoap,OlegHahm/libcoap,PolymorhicCode/libcoap,thecodemaiden/libcoap-fork,OlegHahm/libcoap,joaopedrotaveira/libcoap,fredericgermain/libcoap,alexaring/libcoap,OlegHahm/libcoap,nmacs/libcoap,sgso/libcoap,pabigot/libcoap,fredericgermain/libcoap,joaopedrotaveira/libcoap,alexaring/libcoap,nmacs/libcoap,sgso/libcoap,fredericgermain/libcoap,gebart/libcoap,gebart/libcoap,authmillenon/libcoap,thecodemaiden/libcoap-fork,pabigot/libcoap,authmillenon/libcoap,nmacs/libcoap,pabigot/libcoap,RIOT-OS/libcoap,joaopedrotaveira/libcoap,alexaring/libcoap,sgso/libcoap,authmillenon/libcoap,gebart/libcoap