ipv4: ARP neigh procfs buffer overflow

If arp_format_neigh_entry() can be called with n->dev->addr_len == 0, then a
write to hbuffer[-1] occurs.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -1304,7 +1304,9 @@ static void arp_format_neigh_entry(struct seq_file *seq,
 		hbuffer[k++] = hex_asc_lo(n->ha[j]);
 		hbuffer[k++] = ':';
 	}
-	hbuffer[--k] = 0;
+	if (k != 0)
+		--k;
+	hbuffer[k] = 0;
 #if defined(CONFIG_AX25) || defined(CONFIG_AX25_MODULE)
 	}
 #endif