net/gcoap: Avoid endless loop on error
net/gcoap: Avoid endless loop on error

The NULL case can not regularly be reached (because regularly
gcoap_register_listener sets thel link_encoder to a default one), but if
it is (eg. because an application unsets its link_encoder to hide a
resource set at runtime), the existing `continue` is a good idea (skip
over this entry) but erroneously created an endless loop by skipping the
advancement step.

jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OlegHahm/RIOT