ipv4: Put proper checks into icmp_socket_deliver().

All handler->err() routines expect that we've done a pskb_may_pull()
test to make sure that IP header length + 8 bytes can be safely
pulled.

Reported-by: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>