There is no practical way to generate an underlength ARP packet since
an ARP packet is always padded up to the minimum Ethernet frame length
(or dropped by the receiving Ethernet hardware if incorrectly padded),
but the absence of an explicit check causes warnings from some
analysis tools.

Fix by adding an explicit check on the I/O buffer length.

Signed-off-by: Michael Brown <mcb30@ipxe.org>