Fix IP address printing: 4 hex digits per piece

[klao]

Fixes a printing mistake: pieces are uint16, not uint8.

Before:

$ ./ip_address 1.1.1.1
0x101101
$ ./ip_address ::1
0x0000000000000001
$ ./ip_address 2a00:1450:400a:808::200e
0x2A001450400A808000000200E

After:

$ ./ip_address 1.1.1.1
0x01010101
$ ./ip_address ::1
0x00000000000000000000000000000001
$ ./ip_address 2a00:1450:400a:808::200e
0x2A001450400A0808000000000000200E

[foonathan]

Thanks!