shell/gnrc_icmpv6_echo: set success state if DNS query succeeded
shell/gnrc_icmpv6_echo: set success state if DNS query succeeded

We need to set `res` to 0 to signal success, otherwise we end up
in the print usage case.

miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,yogo1212/RIOT,authmillenon/RIOT,jasonatran/RIOT,yogo1212/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,smlng/RIOT,smlng/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,basilfx/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,basilfx/RIOT,miri64/RIOT,basilfx/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,yogo1212/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,ant9000/RIOT,smlng/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,miri64/RIOT,smlng/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT