dns: remove redundant zero length namelen check

The zero namelen check is redundant as it has already been checked
for zero at the start of the function.  Remove the redundant check.

Addresses-Coverity: ("Logically Dead Code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>