CVE: net: fix unbounded memcpy of UDP packet

This patch adds a check to udp_len to fix unbounded memcpy for
CVE-2019-14192, CVE-2019-14193 and CVE-2019-14199.

Signed-off-by: Cheng Liu <liucheng32@huawei.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reported-by: Fermín Serna <fermin@semmle.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>