sock_dns: make sock_dns_query() return the length of the address

The implementation already did that, now also reflect this in the
documentation.

sys/include/net/sock/dns.h

@@ -80,8 +80,8 @@ typedef struct {
  * @param[out]  addr_out        buffer to write result into
  * @param[in]   family          Either AF_INET, AF_INET6 or AF_UNSPEC
  *
- * @return      0 on success
- * @return      !=0 otherwise
+ * @return      the size of the resolved address on success
+ * @return      < 0 otherwise
  */
 int sock_dns_query(const char *domain_name, void *addr_out, int family);
 

sys/shell/commands/sc_gnrc_icmpv6_echo.c

@@ -178,7 +178,7 @@ static int _configure(int argc, char **argv, _ping_data_t *data)
             data->hostname = arg;
 #ifdef MODULE_SOCK_DNS
             if (strchr(data->hostname, ':') == NULL &&
-                sock_dns_query(data->hostname, &data->host, AF_INET6) == 0) {
+                sock_dns_query(data->hostname, &data->host, AF_INET6) > 0) {
                 res = 0;
                 continue;
             }