ping: prevent NULL pointer dereference on write to msg_name

[ Upstream commit cf970c002d270c36202bd5b9c2804d3097a52da0 ]

A plain read() on a socket does set msg->msg_name to NULL. So check for
NULL pointer first.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Conflicts:

	net/ipv4/ping.c