ipv4: avoid passing NULL to inet_putpeer() in icmpv4_xrlim_allow()

ipv4: avoid passing NULL to inet_putpeer() in icmpv4_xrlim_allow()

inet_getpeer_v4() can return NULL under OOM conditions, and while
inet_peer_xrlim_allow() is OK with a NULL peer, inet_putpeer() will
crash.

This code path now uses the same idiom as the others from:
3e3c470dd0c241a0609c5fed912e6cc901a4ea2c ("inet: Minimize use of
cached route inetpeer.").

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>