udpv6: Check address length before reading address family

udpv6: Check address length before reading address family

KMSAN will complain if valid address length passed to udpv6_pre_connect()
is shorter than sizeof("struct sockaddr"->sa_family) bytes.

(This patch is bogus if it is guaranteed that udpv6_pre_connect() is
always called after checking "struct sockaddr"->sa_family. In that case,
we want a comment why we don't need to check valid address length here.)

Signed-off-by: Tetsuo Handa <[email protected]>
Acked-by: Song Liu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>