net: ipv6: Fix CID 453851 and CID 436278

CID 453851 : sprintf() shouldn't copy from/to tmp
CID 436278 : DHCP6 option_len should be checked before use

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>