Invalid Read in DHCP

DHCP can read past the end of the packet:

==28122== Thread 3:
==28122== Invalid read of size 2
==28122==    at 0x402D558: memcpy (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==28122==    by 0x805EDCA: ip_addr_init (ec_inet.c:74)
==28122==    by 0x80754C9: dissector_dhcp (ec_dhcp.c:156)
==28122==    by 0x8059AF8: decode_data (ec_decode.c:304)
==28122==    by 0x808C448: decode_udp (ec_udp.c:129)
==28122==    by 0x8089884: decode_ip (ec_ip.c:230)
==28122==    by 0x8088C66: decode_eth (ec_eth.c:81)
==28122==    by 0x8059793: ec_decode (ec_decode.c:186)

Example file: https://www.wireshark.org/download/automated/captures/fuzz-2012-11-26-27661.pcap

I simply added a check to ensure that we don't read too far. There are other place in DHCP where I believe this can occur, but I'd rather have concrete examples before addressing them. Probably a good place to start with unit tests.