tcp: Silence warning: ‘in’ may be used uninitialized

Initialise 'in' to avoid...

net/ipv4/tcp.c: In function 'tcp_nuke_addr':
net/ipv4/tcp.c:3208:11: warning: 'in' may be used uninitialized in this function [-Wmaybe-uninitialized]
     if (in->s_addr != s4 &&
           ^

Signed-off-by: Jon Medhurst <tixy@linaro.org>